2 Tbsp olive oil
4 cups thinly sliced onions (sliced crosswise, see how to slice onions)
6 medium Yukon gold potatoes, peeled, thinly sliced (1/8-inch thick, it helps to use a mandoline)
1/3 cup stock (chicken, beef, or vegetable stock, use vegetable stock for vegetarian option)
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
1 teaspoon of butter to butter the gratin or casserole dish
1 teaspoon finely minced fresh rosemaryDirections
1 Heat olive oil on medium high heat in a thick-bottomed sauté pan. Add the sliced onions, stir to coat with the olive oil, and cook until golden brown, about 15 to 20 minutes.
2 Preheat oven to 400°F (205°C). In a large bowl put the sliced potatoes and gently toss with 1 Tbsp of olive oil, 1/2 cup grated Gruyere cheese, 1/2 cup of grated Parmesan, salt, and white pepper.
3 Butter a large gratin dish or 9x13 casserole lightly and line with the caramelized onions in an even layer at the bottom of the dish. Arrange the sliced potatoes (including the grated cheeses) in a pattern over the onions. Pour stock over the potatoes. Sprinkle with remaining cheese.
4 Cover with aluminum foil and bake at 400°F (205°C) for 50 minutes (or until fork tender). Remove aluminum foil. Broil for 5 minutes, uncovered, until lightly browned.
Sprinkle with minced rosemary to serve.