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 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 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 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. DO AHEAD: Can be made 2 hours ahead. Let stand at room temperature. Tent with foil and rewarm in a 300° oven until hot, about 20 minutes.
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