arrange ⅓ of the potatoes in the dish, overlapping them. layer ½ of the onion slices evenly over the potatoes. Pour ⅓ cup whipping cream over the layer, and spread ½ the Gruyere cheese.
Repeat the process in the step above.
Arrange the remaining potatoes evenly over the top, and pour over the last ⅓ cup whipping cream, pressing the potatoes down so they are submerged in the liquid. Arrange the cheddar cheese over the top.
Bake 1 hr 15 minutes, until potatoes are tender and top is starting to brown. Remove from oven and garnish with parsley.
Let stand 15 minutes before slicing and serving.
Because cheese is salty, I haven't added and salt to the recipe, but if you prefer to, sprinkle salt and pepper on each layer of potatoes.Additions such as crumbled bacon, diced ham, mushrooms, or other veggies are delicious in this dish.