We are talking calorie-busting decadence here, but oh so worth it! Just keep in mind, a little goes a long way—think French when considering portion size; after all, Gruyère cheese is in the gratin. And it was a perfect accompaniment to our main course of sous vide, pan-seared beef tenderloin, which in of itself is very lean.
The Hubs and I were a bit leery about the seemingly low amount of liquid, but it turned out perfectly creamy. One of our dinner guests admitted to disliking celery immensely and was concerned when he heard it contained celery root. However, he loved the gratin, even taking seconds!
A mandoline comes in real handy for uniformly-thin slices of potatoes and celery root. To aid in scheduling your dinner, this gratin can be made up to two hours ahead of time, covered with foil, then reheated for about 20 minutes.
Potato, Celery Root and Leek Gratin with Gruyere
- 3 cups heavy cream
- 2 garlic cloves, peeled
- 1 sprig thyme plus 3 tsp. fresh thyme leaves, divided
- 2 Tbsp. unsalted butter, divided
- 3 leeks, white and pale-green parts only, halved lengthwise, thinly sliced crosswise
- Kosher Salt
- 2 lbs. russet potatoes, peeled, very thinly sliced crosswise (1/8″ thick)
- 1 lb. celery root, peeled, very thinly sliced crosswise (1/8″ thick)
- 2 cups grated Gruyère
- Freshly ground black pepper
- Preheat 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; 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.