- 2 pounds Yukon Gold potatoes
- Kosher salt and freshly ground pepper
- 1 tablespoons olive oil
- 1 yellow onion, diced
- 1 1/2 cups heavy cream
- 1 cup shredded Gruyère cheese
- 3/4 cup parmesan cheese
- 1/4 cup crème fraîche
- Fresh thyme for garnish
- Preheat an oven to 400°F. Grease a 9-inch baking dish or large oven proof skillet with butter or non-stick cooking spray.
- Using a mandoline, cut the potatoes into slices, 1/8 inch thick. Arrange the potato slices, slightly overlapping, on the bottom of the baking dish and season with salt and pepper.
- In a skillet over medium heat, heat the olive oil. Add the onion and sauté until starting to caramelize, about 6-7 minutes. Add the cream and bring to a simmer. Simmer until thickened slightly, about 5 minutes, making sure to stir to prevent sticking. Stir in 3/4 cup of the gruyere cheese and all of the parmesan and cook until melted. Remove the mixture from the heat and stir in the crème fraîche. Season with salt and pepper.
- Pour the cream mixture evenly on top of the potatoes and sprinkle the remaining 1/4 cup of Gruyere cheese evenly on top. Bake until the mixture is golden brown and bubbly and the potatoes are tender, about 1 hour. Let the gratin rest for about 15 minutes. Garnish with thyme and serve immediately.