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.