- 2 Tbsp butter, at room temperature, plus more for greasing
- 4 shallots, thinly sliced
- 2 cloves garlic, thinly sliced
- Salt and freshly ground white pepper
- 2 celery root, peeled, halved, and thinly sliced
- 2 lb russet potatoes peeled and thinly sliced
- 1 Tbsp chopped fresh thyme
- 2 cups heavy cream
- 1 cup low sodium chicken broth
- 1 1/2 cups grated Gruyere cheese
Preheat oven to 375°.
Butter a 14” gratin dish. Sprinkle half of shallots and garlic on bottom of dish and season with salt and pepper. Arrange half of celery root and potato slices on top of shallots, sprinkle half of thyme on top, and season with salt and pepper. Repeat layering with remaining shallots, garlic, celery root, potatoes, and thyme, and season with salt and pepper.
In a saucepan over medium-low heat, bring cream and broth to a simmer and pour over vegetables. Dot 2 tablespoons butter on top and cover tightly with aluminum foil.
Bake until vegetables are almost tender when pierced with a fork, about 35 minutes. Increase temperature to 400°. Press vegetables down with a spatula to an even thickness. Remove from oven, uncover, sprinkle with cheese and continue to bake until gratin is golden brown, about 30 minutes. Let stand for 5 minutes before serving.