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 (or black pepper if that is what you have)
1 teaspoon of butter to butter the gratin or casserole dish
1 teaspoon finely minced fresh rosemary