3/4 cup (packed) extra -sharp cheddar cheese (about 4 ounces)
3/4 cup (packed) Gruyere cheese, grated
1/3 cup (packed) freshly grated Parmesan cheese (about 1 1/4 ounces)
4 pounds russet potatoes, peeled, cut into 1/4 inch thick rounds
1 1/2 teaspoons salt
1/2 teaspoon ground black pepper
1/4 cup finely chopped onion
3 tablespoons all purpose flour
4 tablespoons (1/2 stick) butter
3 cups whole milk
1. Preheat oven to 400 degrees. Lightly butter a 13×9 inch glass baking dish. Mix cheddar cheese, gruyere cheese and Parmesan in small bowl.
2. Arrange half of potatoes in prepared baking dish, overlapping slightly. Sprinkle with teaspoon salt and 1/4 teaspoon pepper. Sprinkle onion over, then flour. Dot with 2 tablespoons butter. Sprinkle half of cheese mixture over. Top with Remaining potatoes, 3/4 teaspoon salt, 1/4 teaspoon pepper and 2 tablespoons butter. Reserve remaining cheese.
3. Bring milk to simmer in medium saucepan. Pour milk over potatoes (milk will not cover potatoes completely). Cover baking dish tightly with foil. Bake 45 minutes. Uncover dish; sprinkle potatoes with reserved cheese mixture. Bake uncovered until potatoes are tender and cheese is deep golden brown, about 45 minutes longer. Remove from oven; let stand 15 minutes before serving.