This Potato, Mushroom, Caramelized Onion Gratin is rich, satiny, silky, and utterly decadent. It’s a beautiful special occasion side dish.
Potato Mushroom Caramelized Onion Gratin
I stole the recipe from my StepMom, Pat, who served it for years every Christmas dinner. She doesn’t know I fabulously frugalized it and served it for my holiday dinners, too.
About Potato, Mushroom, Caramelized Onion Gratin:
I’m so happy to share my revised version with you today. Whatever you do, don’t tell Pat! Shhh!
This dish is layer upon layer of decadence: potatoes, caramelized onions, and mushrooms with lots and lots of cheese are all but swimming in a beautiful, creamy sauce that’s kissed with a touch of wine.
Topped with just a bit more cheese, it’s baked until bubbly and golden and crispy on top and it’s just a beautiful thing. Once, I accidentally left it “too long” in the oven. It was a happy mistake. Too long” turned the cheese on top into an incredibly caramelized crispiness. Now I do it that way ALL the time.
Serving:
These potatoes are great for special occasion dinners. While most au gratin potatoes are associated with ham, all this richness gives more of a Steakhouse vibe. For other favorite potato gratins more suitable with Ham, check out Ruth’s Chris Potatoes au Gratin (shown below) or Old Fashioned Scalloped Potatoes.
- The complexity of these potatoes is better paired with beef. Think good steak, ribeye, maybe filet mignon, or a nice beef roast.
- Items that are simply cooked, with just garlic and a few earthy herbs (think rosemary or thyme) won’t compete with the potatoes. Think pork chops or roast, the same with lamb, or sauteed chicken breast.
- You can’t go wrong with a green vegetable as a second side, especially those with stronger flavors like greens or Brussels sprouts.
Ruth’s Chris Potatoes au Gratin Copycat = see how creamy!
Cooking the Mushrooms & Onions:
First, the mushrooms and onions have to be prepared so pay attention! 🙂 I have a special method that speeds things along and works beautifully.
- After the butter is melted (medium-high) and the mushrooms and onions are added, add a lid to the pan.
- Check every couple of minutes: lift the lid and stir. If the bottom of the pan is getting dry and dark, add a tablespoon or so of water, stir, and scrape any residue from the bottom of the pan. Repeat until the mushrooms are turning golden and the onions are softened.
- Remove the lid, again adding a little water if needed, and cook, stirring often for two or three minutes until all are beautifully caramelized. Then drop in the garlic and thyme and cook until the garlic loses its sharp bite.
- Don’t forget to taste them! Add salt if needed.
Notes on Baking the Potato, Mushroom, Caramelized Onion Gratin:
The rest of the recipe is straightforward. Do pay attention to the layering.
The dish is ready to be topped with cheese when it’s bubbly and a knife inserted in the center meets no resistance. Bake another twenty minutes or so until the cheese is golden brown, running under the broiler if necessary. We prefer it darker; it will become crispy and extra flavorful.
Right out of the oven the dish will be very liquidy. Set it aside for about 20 minutes; the potatoes and saucy liquid will meld together.
Double the recipe if desired, but allow more cooking time and use a very large pan or divide into two pans.
We love to cook it so the top is dark and crispy
Storing and Reheating:
Store covered in the fridge for four to five days. Reheat in the microwave, being careful not to overheat areas, which can cause the cream to separate. Smaller amounts reheat best.
Typically recipes with cream do not freeze well and the texture of mushrooms can suffer.
The Ingredients:
- Potatoes: Stick with Russet. They have enough starch to meld with the liquid in the recipes and come out beautifully, just holding together but almost melting. Lower starch potatoes will remain more distinct, but the sauce will be too thin.
- Mushrooms: Most mushrooms will work; I chose buttons because they’re well-liked and inexpensive. Go higher end if you wish – Pat did.
- Cheese: The original recipe calls for gruyere but feel free to sub in our American Swiss, white Cheddar, Provolone, or a combination of all three. Stay away from Orange cheeses – they don’t complement the caramelized onions and mushrooms.
- Herbs: We love this with thyme but options might be marjoram or rosemary.
Saving Money on Potato, Mushroom, Caramelized Onion Gratin:
As far as the change-ups: I halved the recipe, subbed in caramelized onions instead of leeks, and used budget-minded button mushrooms. Take advantage of holiday sales (see Win at the Grocers for links of items to expect to be on sale before a holiday) and discount stores like Aldi or Lidl for all these ingredients.
Mushrooms:
Whole mushrooms keep better. Store by poking a hole or two in the plastic on top, then poking several holes in the bottom; place upside down in the fridge. This reduces condensation and buys a couple of days.
- Mushrooms are often 1/2 off before holidays. Discount store’s everyday pricing rivals grocery stores’ best sales. Buyer’s clubs aren’t the cheapest option but their mushrooms are always gorgeous.
- The bargain is the button; its nutritional value is nearly identical to its pricier cousins.
Russets:
Russets are dirt cheap, but there are some tricks. Once home, store in a dark, cool, dry area, well-ventilated and away from onions. Remove from plastic. A paper grocery bag with the top folded over isn’t elegant but keeps them in the dark and reduces condensation.
- Think seasonally; although never expensive, russets from one variety or another are in season and at their lowest from late summer through winter. Take advantage of that lower pricing to use them more often.
- For best value, the larger bags will be cheaper per pound unless there’s a great sale on another size.
- The largest bags might have a mix of sizes; sort and pick out larger, more uniform ones for recipes needing whole potatoes, reserving others for recipes like this one where size doesn’t matter.
Cream:
- Cream is always cheaper at discount groceries. Their everyday price beats out the buyer’s club and the best grocery store holiday sales price.
- Cream keeps for weeks so if the standard grocery is the only option, get in the habit of picking at a low before holidays.
Grocery Cheese:
- Grocery Store Cheese is a great item to pick up at rock bottom and keep on hand. The best pricing is a discount store’s sales, but if not an option, know the lows at your grocery.
- Cheese keeps for weeks, unopened, and can be frozen. After freezing, it will be a little crumbly but still great for casseroles and cooking.
Potato Mushroom Caramelized Onion Gratin – so silky and tasty.
Potato Mushroom & Caramelized Onion Gratin
- Prep Time: 15 minutes
- Total Time: about 2 hours
- Yield: 6 to 8 servings 1x
- Category: Side
- Cuisine: American
Ingredients
- 2 tablespoons butter plus extra to butter casserole
- 8 ounces mushrooms, sliced
- 1 large onion, sliced about 3/8ths inch thick, pole to pole
- 2 large cloves garlic, minced
- 1 1/2 teaspoon fresh thyme or 1/2 teaspoon dried
- salt to taste for onion/mushrooms
- 3/4 cups chicken broth
- 1 cup whipping or heavy cream
- 2 tablespoons dry white wine
- 1 teaspoon salt
- 1/4 teaspoon ground black pepper
- 2 1/4 pounds russet potatoes, peeled, thinly sliced into rounds (divide into thirds)
- 2 1/4 cups grated Gruyère or Swiss cheese (about 10 ounces) (divide into thirds)
Instructions
Prepare the Ingredients:
Preheat oven to 400°F. Butter a 2-quart baking dish.
Melt the 2 tablespoons butter in heavy large skillet over medium-high heat. Add mushrooms and onions and gently sauté until with the lid on, about eight minutes, until the mixture is tender and starting to brown. Check now and then and stir, adding a little water if they seem dry. There’s no hard and fast rule as to how much water, just add a tablespoon or two, stir up any color off the bottom of the pan, and add the lid again.
Once the mushrooms are turning golden brown and the onions are softened, remove the lid and continue to cook, stirring now and then, until excess liquid has evaporated. Continue to cook a minute or two longer until the mushrooms and onions are dark golden. Add garlic and thyme and heat until the garlic is fragrant and has lost its raw taste. It’s ok to taste; make sure the garlic is cooked through and add a little salt if needed. (May be made 1 day ahead. Cover and refrigerate.)
While the mushroom-onion mixture is cooking, prepare your potatoes and set aside. In a large bowl, whisk broth, cream, wine, salt, and pepper to blend and set aside.
Layer the casserole:
Use 1/3 of the potatoes as the bottom layer, top with half the mushroom mixture, and about 3/4 cup of cheese. Pour half the cream mixture over the top.
Add a second layer of potatoes, the rest of the mushroom mixture, and another 3/4 cup of cheese. Top with the last layer of potatoes and pour the remaining cream mixture over the top. Reserve the remaining cheese.
Bake & Serve:
Cover loosely (a piece of foil just laid over the top is ideal) and bake until potatoes are just tender and the sauce is bubbly, about an hour. Check by piercing the center with a knife. There should be no resistance.
Top with the remainder of the cheese and bake uncovered for another 15 to 20 minutes until the cheese is golden brown. If necessary, the dish can be run under the broiler to brown.
Straight from the oven, the gratin will seem very liquidy. Let stand 20 to 30 minutes before serving; the texture will be perfect.
Notes:
- For maximum browning, use a shallow au gratin pan.
- Place a sheet of foil on the next rack below the potatoes to catch any drips.
- Make sure to allow time to let the potatoes stand after removing from the oven.
Making Ahead:
There are two options for making ahead, either assembling the day before and baking the next day or partially baking the day before and finishing the next.
- Assemble a day ahead (no longer, and without the final topping of cheese) and bake the next day. Ensure all potatoes are covered in sauce to prevent discoloration, then place plastic wrap directly on top and cover the dish tightly with foil. When ready to bake, remove from fridge, remove the foil, and let sit at room temperature for an hour. Add a bit of extra baking time to ensure they cook through. Add the top layer of cheese as directed.
- For better results, par-bake: Cover with foil and bake at 350°F for about 45–50 minutes, or until the potatoes are just tender but not fully cooked. Allow to cool, cover it tightly with foil or plastic wrap, and refrigerate. When ready to bake, remove the dish from the fridge, uncover, and let it sit at room temperature for 30 minutes. Place in a 350°F oven and bake, covered lightly with foil, for 25 to 30 minutes, or until bubbly, golden, and fully cooked. Top with cheese and continue.
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I’m sharing Potato, Mushroom, Caramelized Onion Gratin at Fiesta Friday, hosted by Ahila @ A Taste of Sri Lankan Cuisine and Diann @ Of Goats and Greens.
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