Preheat the oven to 400°F.

Heat 1 tablespoon of 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.

Put the sliced potatoes in a large bowl and gently toss with 1 tablespoon of olive oil, 1/2 cup grated Gruyere cheese, 1/2 cup of grated Parmesan, salt, and white pepper.

Butter a large gratin dish or 9 x 13-inch casserole lightly and line with the caramelized onions in an even layer at the bottom of the dish.

Arrange the sliced, cheese-coated potatoes in a pattern over the onions.

Pour stock over the potatoes. Sprinkle with remaining cheese.

Cover with aluminum foil and bake for 50 minutes, or until fork tender.

Remove aluminum foil. Broil for 5 minutes, uncovered, until lightly browned.

Sprinkle with minced rosemary to serve.