Celeriac and Potato Gratin with Leeks
- 3 cups heavy cream
- 2 garlic cloves, peeled
- 1 sprig thyme plus 3 teaspoons fresh thyme leaves, divided
- 2 tablespoons (1/4 stick) unsalted butter, divided
- 3 leeks, white and pale-green parts only, halved lengthwise, thinly sliced crosswise
- Kosher salt
- 2 pounds russet potatoes, peeled, very thinly sliced crosswise (1/8" thick)
- 1 pound celery root, peeled, very thinly sliced crosswise (1/8" thick)
- 2 cups grated Gruyère
- Freshly ground black pepper
Preheat oven to 350°F. Heat cream, garlic, and thyme sprig in a medium saucepan just until bubbles begin to form around edge of pan. Remove from heat; set aside to steep. Melt 1 tablespoon butter in a medium skillet over medium heat. Add leeks; season with salt and cook, stirring often, until tender (do not brown), 10-12 minutes. Transfer to a small bowl and set aside.
Butter a 3-quart gratin dish with remaining 1 tablespoon butter. Layer ⅓ of potato slices and ⅓ of celery root slices evenly over bottom of baking dish. Cover with ⅓ of leeks, then ⅓ of Gruyère. Sprinkle with salt, pepper, and 1 teaspoon thyme leaves. Repeat layers twice more. Strain cream mixture into a medium pitcher and pour over vegetables. Set gratin dish on a large rimmed baking sheet and cover tightly with foil. Bake for 1 hour. Carefully remove foil; continue baking until top is golden brown and sauce is bubbling, 25-30 minutes.