1. Preheat oven to 425ºF. Grease a 9-inch round baking dish.
2. Arrange half of onion slices in a circular pattern in dish, overlapping slightly. Repeat with potato slices, then sprinkle with 1/2 tsp. salt, 1/2 tsp. pepper and 1/2 cup Gruyère. Drizzle with 2 Tbsp. butter. Repeat with remaining ingredients, finishing with a top layer of potato slices.
3. Bring milk to boil and carefully pour over potatoes. Bake until tender and golden, 45 to 55 minutes.