Potato & Celery Root 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 Gruyere
• Freshly ground black pepper

1. Preheat oven to 350u0b0F. 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.
2. 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.
3. Butter a 3-quart gratin dish with remaining 1 tablespoon butter. Layer 1/3 of potato slices and 1/3 of celery root slices evenly over bottom of baking dish. Cover with 1/3 of leeks, then 1/3 of Gruyere. Sprinkle with salt, pepper, and 1 teaspoon thyme leaves. Repeat layers twice more. Strain cream mixture into a medium pitcher and pour over vegetables.
4. 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. DO AHEAD:

heavy cream, garlic, thyme, unsalted butter, leeks, kosher salt, russet potatoes, celery root, gruyere, freshly ground black pepper

Taken from www.epicurious.com/recipes/food/views/potato-celery-root-gratin-with-leeks-368278 (may not work)