Warm Onion-Potato Gratin

Preparation Combine Yukon Gold potatoes and sliced onions in heavy large saucepan. Add enough water to cover. Bring water to boil. Reduce heat and simmer until potatoes are almost tender, about 3 minutes. Drain potato-onion mixture well. Arrange half of potato-onion mixture in 11x7-inch glass baking dish. Sprinkle with salt and pepper. Sprinkle mixture with 1/3 cup Gruy&ère cheese, then 2 tablespoons Parmesan cheese. Arrange remaining potato-onion mixture atop cheeses. Pour cream over. Sprinkle with salt and pepper, then remaining 2/3 cup Gruy&ère cheese and 6 tablespoons Parmesan cheese. (Can be prepared 8 hours ahead. Cover and refrigerate.) Preheat oven to 400°F. Bake gratin uncovered until cream thickens, about 25 minutes. Remove from oven. Preheat broiler. Broil gratin until top is golden, about 2 minutes.

Preparation Combine Yukon Gold potatoes and sliced onions in heavy large saucepan. Add enough water to cover. Bring water to boil. Reduce heat and simmer until potatoes are almost tender, about 3 minutes. Drain potato-onion mixture well. Arrange half of potato-onion mixture in 11x7-inch glass baking dish. Sprinkle with salt and pepper. Sprinkle mixture with 1/3 cup Gruy&ère cheese, then 2 tablespoons Parmesan cheese. Arrange remaining potato-onion mixture atop cheeses. Pour cream over. Sprinkle with salt and pepper, then remaining 2/3 cup Gruy&ère cheese and 6 tablespoons Parmesan cheese. (Can be prepared 8 hours ahead. Cover and refrigerate.) Preheat oven to 400°F. Bake gratin uncovered until cream thickens, about 25 minutes. Remove from oven. Preheat broiler. Broil gratin until top is golden, about 2 minutes.