Caramelized Onion and Gruyère Potato Gratin with Fresh Thyme
45min
1h 30min
10
485
Chef's Note
For the most elegant presentation, use a mandoline to achieve uniform potato slices of exactly 1/8-inch thickness. This ensures even cooking and a beautiful layered effect. You can prepare this gratin up to 24 hours in advance and refrigerate before baking - just add an extra 15 minutes to the covered baking time.
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Ingredients
5 pounds Yukon Gold potatoes
3 whole large sweet onions
4 tablespoons unsalted butter
2 tablespoons olive oil
3 tablespoons fresh thyme leaves
4 sprigs fresh thyme sprigs (for garnish)
2 cups Gruyère cheese, finely grated
1 cup Parmigiano-Reggiano, finely grated
3 cups heavy cream
1 cup whole milk
4 cloves garlic cloves, minced
1/4 teaspoon nutmeg, freshly grated
2 leaves bay leaves
2 teaspoons kosher salt
1 teaspoon white pepper, freshly ground
1 tablespoon Dijon mustard
Instructions
- 1
Preheat oven to 350°F (175°C) and position a rack in the middle. Butter a 9x13-inch baking dish with 1 tablespoon of butter.
- 2
Thinly slice the onions into half-moons. In a large skillet over medium-low heat, melt 2 tablespoons of butter with 2 tablespoons of olive oil. Add the sliced onions with 1/2 teaspoon of salt and cook for 35-40 minutes, stirring occasionally, until deeply caramelized and golden brown. Add 1 tablespoon of thyme leaves in the last 5 minutes of cooking.
- 3
While the onions caramelize, peel the potatoes and slice them 1/8-inch thick using a mandoline or sharp knife. Place sliced potatoes in a large bowl of cold water to prevent browning.
- 4
In a medium saucepan, combine 3 cups of heavy cream, 1 cup of milk, 4 minced garlic cloves, 2 bay leaves, 1/4 teaspoon freshly grated nutmeg, 1 teaspoon of salt, and 1 teaspoon of white pepper. Bring to a gentle simmer over medium heat, then reduce heat to low and simmer for 10 minutes to infuse flavors. Remove from heat, discard bay leaves, and whisk in 1 tablespoon of Dijon mustard.
- 5
Drain the potatoes thoroughly and pat dry with paper towels. In a large bowl, toss the potato slices with 1/2 teaspoon of salt and remaining 2 tablespoons of fresh thyme leaves.
- 6
Layer 1/3 of the potato slices in the prepared baking dish, slightly overlapping them. Sprinkle with 1/3 cup of Gruyère and 1/4 cup of Parmigiano-Reggiano. Spread half of the caramelized onions over the cheese.
- 7
Add another 1/3 of the potato slices, 1/3 cup of Gruyère, 1/4 cup of Parmigiano-Reggiano, and the remaining caramelized onions.
- 8
Arrange the final layer of potato slices on top, then pour the infused cream mixture evenly over the gratin. Press down gently with a spatula to ensure potatoes are mostly submerged.
- 9
Sprinkle the remaining Gruyère and Parmigiano-Reggiano over the top. Cover the dish with aluminum foil and place on a rimmed baking sheet to catch any overflow.
- 10
Bake covered for 45 minutes, then remove foil and bake for an additional 35-45 minutes until the top is golden brown and bubbling, and potatoes are fork-tender.
- 11
Let the gratin rest for 15-20 minutes before serving to allow the layers to set. Garnish with fresh thyme sprigs before serving.
Nutrition Information (per serving)
485
18g
36g
32g
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