POTATO, CELERY ROOT AND LEEK GRATIN (for eight)
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
2 pounds russet potatoes, peeled, very thinly sliced crosswise (1/8 inch)
1 pound celery root, peeled, very thinly sliced crosswise (1/8 inch)
2 cups grated Gruyere
freshly ground black pepper
1. Preheat oven to 350*. Heat cream, garlic and thyme sprig in a medium saucepan just until bubbles begin to form around edge of pan. Remove from heat and 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 to 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 potatoes 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, 30 to 35 minutes, more if needed to cook the celery root completely through.
NOTE: 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.
from Bon Appetit magazine, November, 2011