Scalloped Potatoes with Caramelized Onions and Gruyere

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 rosemary

Directions

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 9×13 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.