Scalloped Potatoes

1 large onion, halved and thinly sliced
2 1/2 lb red potatoes, peeled and sliced
1 tsp salt, divided
1 tsp pepper, divided
1 cup shredded Gruyere, divided
4 tbsp unsalted butter, melted
1 1/4 cups milk

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