- Unsalted butter, for the dish
- 4 shallots, thinly sliced
- 3 medium bulbs celeriac
- 2 sprigs fresh thyme
- 1 cup heavy cream
- 1 tablespoon Dijon mustard
- Freshly grated nutmeg
- Salt and freshly ground black pepper
- 3/4 cup freshly grated Gruyere cheese
- 1/4 cup freshly grated Parmesan cheese
- Heat oven to 400 degrees. Butter a 1 1/2-quart gratin dish. Scatter shallots over bottom of dish. Trim celeriac by cutting off bottom and top, then cutting off thick outer layer with a sharp knife. Cut into 1/4-inch slices, and then julienne. Arrange evenly in gratin dish. Sprinkle thyme leaves over celeriac.
- In a small bowl, whisk together cream, mustard, nutmeg, and salt and pepper. Pour over celeriac, and sprinkle with cheeses. Cover with foil, and bake for 20 minutes.
- Remove foil, and continue baking until top is brown and bubbly and cream is thickened and reduced, about 20 more minutes. Let cool for 10 minutes, and serve.