Unsalted butter, for the dish
1 yellow onion, 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
1Heat oven to 400 degrees. Butter a 1 1/2-quart gratin dish. Scatter onions over bottom of dish.
2 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.
3 In a small bowl, whisk together cream, mustard, nutmeg, and salt and pepper. Pour over celeriac, and sprinkle with cheeses.
4 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.