Preheat oven to 425 degrees F.
Melt the butter, and sauté the onion over medium heat until soft and translucent. Add the garlic, salt, and pepper and cook for about 30 seconds. Add the stock, cream, and potatoes and bring to a good (not a boil) simmer. Cover, reduce heat to medium-low and simmer gently until the potatoes are nearly tender when pierced with a knife, about 15 to 20 minutes. Taste and adjust seasoning to your taste. Gently stir.
Transfer the mixture to a buttered 8 x 8″ or equivalent sized baking dish (or about five to six individual casseroles). Top with the cheese and bake in the preheated oven for about 10 -15 minutes for the casserole (a little less for individual casseroles) or just until the sauce is bubbling and the cheese is melted. Cool a few minutes before serving. Sprinkle with chopped parsley.
Note: Although our family loves this just as is, this is a very creamy dish with cream, potatoes and cheese, and just a little onion, one clove garlic and the bit of chicken stock for additional flavor. It’s fine to taste the sauce before you transfer to the casserole and be sure to adequately season with salt and pepper. If you are a person who prefers a dish that is more highly flavored, feel free to increase the onion and/or garlic to taste.
Keywords: au gratin potatoes, cheddar, Cheese, Chicken Stock, Cook's Illustrated, Cream, fontina, hearty sides, parmesan, Potatoes, provolone, Ruth's Chris, Side, Steakhouse