• 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
Until the onions are golden brown, sauté them as follows: In a thick-bottomed sauté pan, heat 1 tablespoon olive oil over medium high heat. Add the sliced onions, toss to coat with the olive oil, and simmer for 15 to 20 minutes, or until golden brown.
Preheat the oven to 400 degrees Fahrenheit (205 degrees Celsius).
Toss sliced potatoes with olive oil, cheeses, salt, and pepper in a large mixing bowl: Toss the sliced potatoes with 1 tablespoon olive oil, 1/2 cup grated Gruyere cheese, 1/2 cup grated Parmesan, salt, and white pepper in a large mixing dish.
(If you’re making ahead of time, make sure the potatoes are properly coated in olive oil, even if you have to add more.) This will keep them from becoming grey.)
In a buttered dish, layer onions, stack potatoes on top, pour stock over, and top with additional cheese: In a large gratin dish or 913 casserole, lightly butter the bottom and put the caramelized onions in a uniform layer.
Arrange the sliced potatoes in a pattern over the onions, including the grated cheeses. Over the potatoes, pour the stock. The remaining cheese should be sprinkled on top.
Cover with foil and bake, then finish with a broiler: Cover with aluminum foil and bake for 50 minutes at 400°F (205°C) (or until fork tender). Remove the aluminum foil from the pan. Cover and broil for 5 minutes, or until lightly browned. To serve, top with minced rosemary.