1 1/2 pounds Yukon Gold potatoes, peeled, cut into 1/4 inch thick slices
3/4 pound onions, thinly sliced
1 cup (packed) grated Gruyere cheese (about 4 oz)
8 Tbsp (packed) freshly grated Parmesan cheese (about 2 oz)
2/3 cup whipping cream
Preheat oven to 400°F (unless preparing in advance).
1 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.
2 Arrange half of potato-onion mixture in 11x7 inch glass baking dish. Sprinkle with salt and pepper. Sprinkle mixture with 1/3 cup Gruyere cheese and then 2 Tbsp of Parmesan cheese.
3 Arrange remaining potato-onion mixture atop cheeses. Pour cream over. Sprinkle with salt and pepper. Sprinkle the remaining 2/3 cup of Gruyere and 6 Tbsp of Parmesan cheese. (Can be prepared 8 hours in advance. Cover and refrigerate.)
4 Bake gratin uncovered in 400°F oven until cream thickens, about 25 minutes. Remove from oven. Preheat broiler. Broil gratin until top is golden, about 2 minutes.