Ingredients:

• 2 tablespoons extra virgin olive oil, divided
• 4 cups thinly sliced onions, sliced crosswise
• 6 medium Yukon gold potatoes (about 1 1/2 pounds), peeled and sliced 1/8 inch thick (see recipe note)
• 3/4 cup grated Gruyere cheese, divided
• 1/2 cup grated Parmesan cheese
• 1 teaspoon kosher salt
• 1/2 teaspoon white pepper (or black pepper if that is what you have)
• 1 teaspoon butter, for greasing the baking dish
• 1/3 cup stock (chicken, beef, or vegetable)
• 1 teaspoon finely minced fresh rosemary