Unsalted butter for baking dish
1 Tbs. olive oil
1/2 cup finely chopped shallots
1 tsp. chopped fresh thyme
2 cups heavy cream
1 tsp. Dijon mustard
2 tsp. kosher salt
1/4 tsp. freshly ground pepper
1 1/2 lb. russet potatoes, peeled and sliced 1/16 inch thick
1 1/2 lb. celery root, peeled and sliced 1/16 inch thick
4 oz. Gruyère cheese, shredded
1 oz. Parmigiano-Reggiano cheese, grated
2 tsp. chopped fresh chives or flat-leaf parsley (optional)
Preheat an oven to 375°F. Butter a 13-by-9-inch baking dish.
In a saucepan over medium heat, warm the olive oil. Add the shallots and thyme and cook, stirring occasionally, for 3 minutes. Add the cream, mustard, salt and pepper and whisk until just blended. Remove from the heat and let cool slightly.
In a large bowl, toss together the potato and celery root slices. Pour the cream mixture over the potato mixture and toss to coat evenly.
In a small bowl, stir together the Gruyère and Parmigiano-Reggiano.
Arrange a layer of potato and celery root slices, slightly overlapping, in the prepared baking dish. Sprinkle with one-third of the cheese mixture. Repeat the layering 2 more times, ending with the cheese. Pour any remaining cream from the bowl over the gratin.
Cover the dish loosely with foil and bake for 45 minutes. Remove the foil and continue baking until the top is golden and the vegetables are tender when pierced, 30 to 45 minutes more. Let stand for 10 minutes, then sprinkle with the chives and serve. Serves 8 to 10.