4 medium Yukon gold (1 1/2 pounds), thinly sliced (about 5 cups)
1 small butternut squash (1 1/2 pounds), peeled, halved, seeded, and thinly sliced crosswise
1/2 cup thinly sliced leek (1 large) or green onions (4)
1 tablespoon snipped fresh sage
4 cloves garlic, minced
1 teaspoon salt
1/2 teaspoon ground nutmeg
1/4 teaspoon ground black pepper
8 ounces Fontina cheese, shredded (2 cups)
1 1/2 cups whipping cream
Sauteed leek slices (optional)
Preheat oven to 350 degrees F. Grease a 3-quart baking dish or gratin dish. Layer half of the sliced potatoes, butternut squash, and leeks in the prepared dish. Sprinkle with half of the sage, garlic, salt, nutmeg, and pepper. Sprinkle with half of the cheese. Repeat layers. Pour whipping cream over top. Cover tightly with foil.
Bake, covered, for 80 minutes. Uncover; bake about 30 minutes more or until potatoes are tender when pierced with a fork and top is golden brown. Let stand for 15 minutes before serving. If desired, top with additional sliced leek that has been sauteed in olive oil.