In a saucepan, combine the sliced potatoes, cream or milk, garlic, salt and pepper. Heat this mixture until it is just coming to a boil. Immediately reduce the heat to a gentle simmer. Do not let the cream come to a hard boil. Cook for 5 or 6 minutes, or until the potatoes are almost tender but not fully cooked.
If you are using the cornstarch mixture to thicken the sauce: In a small jar, blend the cornstarch with water. Stir this mixture into the potatoes and simmer until it is thickened, which will be about 1 minute. Remove the pan from the heat and stir in 1/4 cup of the Gruyere cheese plus the Parmesan cheese.
Transfer the potato and cheese mixture to lightly greased 8"x8" (20.5 x 20.5 cm) baking dish. Make sure they are evenly layered.
Sprinkle the remaining grated Gruyere cheese over the potatoes. Bake in a preheated 350ºF (180ºC) oven for 30 to 40 minutes or until the top is golden brown.
Remove from the oven and let stand for 10 minutes before serving.
Notes
Notes: To switch it up a little there are a few optional additions you can make to this elegant potato recipe.
Add 1 cup of sauteed mushrooms. Mix them into the potato mixture just as you are about to place the potatoes into the baking dish.
1/4 teaspoon of chopped fresh rosemary. Add it to the potatoes as you are cooking them. You can add a bit more rosemary but it is a strong herb, so start out with a small amount and add more if you want it.
You can mix 3 tablespoons of bread crumbs (ideally fresh bread crumbs) with the cheese to add to the top of the potato mixture. It may not be quite as traditional, but adds a nice bit of crunch.