Preheat oven to 350 degrees. Toss together potatoes, sweet potatoes, apple, leeks, oil, and 1 teaspoon salt. Season with pepper. Transfer to a 9-by-12-inch oval baking dish, spread into a thin layer, and cover with parchment-lined foil. Bake for 15 minutes. Raise oven temperature to 400. Remove, uncover, and top with Gruyere. Bake until bubbling and top is browned, about 30 minutes.
I have made this recipe several times subsituting based on what I have in my cabinet and it is always delicious. For instance, I always use a mixture of Monterey Jack and parmesan for the gruyere and have substituted yellow onions and gala Apple as needed. A great lower fat version for those who like scalloped potatoes with a delicate sweetness and great blend of flavors and nutrients. What more could you ask for?
Totally delicious! What a terrific combination of flavors and a lighter way to make the classic potato and Gruyere gratin. Definite winner.