Sweet potato side dish

2 lbs sweet potatoes, thinly sliced
1 green apple, cored and grated
3 leeks, white and pale-green parts only, halved lengthwise, sliced 1/8 inch thick, and rinsed well
1 tablespoon extra-virgin olive oil 
Course salt and freshly ground pepper
2 ounces grated gruyere cheese (1 cup)

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.