2 Tbsp olive oil
4 cups thinly sliced onions (sliced crosswise, see how to slice onions)
6 medium-small Yukon gold potatoes (about 1 1/2 pounds), peeled, thinly sliced (1/8-inch thick, it helps to use a mandoline)
3/4 cup grated Gruyere cheese, divided 1/2 cup, 1/4 cup
1/2 cup grated Parmesan cheese
1 teaspoon salt
1/2 teaspoon white pepper (or black pepper if that is what you have)
1 teaspoon of butter to butter the gratin or casserole dish
1/3 cup stock (chicken, beef, or vegetable stock, use vegetable stock for vegetarian option)
1 teaspoon finely minced fresh rosemary
Saute onions until golden: Heat 1 tbsp. olive oil over medium heat in a heavy-bottomed sauté pan.
Add sliced onions, toss to coat with olive oil and cook until golden brown, about 15 to 20 minutes.
Preheat oven to 400°F (205°C).
Toss sliced potatoes with olive oil, cheeses, salt, pepper: In a large bowl, place sliced potatoes and toss gently with 1 tablespoon olive oil, 1/2 cup grated Gruyere, 1/2 cup grated Parmesan, salt and white pepper.
(If you’re going ahead, make sure the potatoes are well coated with the olive oil, even if you need to add more olive oil. This will help keep them from turning gray).
Lay the onions in a buttered dish, arrange the potatoes on top, pour in the broth, sprinkle with more cheese: Lightly butter a large gratin dish or 9×13 pan and lightly line the caramelized onions in an even layer at the bottom of the dish.
Arrange sliced potatoes (including shredded cheese) in a pattern over onions. Pour broth over potatoes. Sprinkle with remaining cheese.
Cover with foil and bake, then broil to finish: Cover with foil and bake at 400°F (205°C) for 50 minutes (or until fork tender). Remove foil. Broil for 5 minutes, uncovered, until lightly browned. Sprinkle with chopped rosemary to serve.