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.
