1 tbs. unsalted butter for baking dish
1 tbs. olive oil
1 cup finely chopped onion
2 cloves garlic, minced
2 sprigs thyme, picked
2 cups heavy cream
1/2 tsp. freshly ground nutmeg
Salt and freshly ground pepper to taste
2 lbs. Yukon Gold potatoes, peeled and sliced 1/8-inch thick
2 lbs. celery root, peeled and sliced 1/8-inch thick
1 cup grated Fontina cheese
1/2 cup grated Parmigiano-Reggiano cheese
Preheat oven to 375 degrees. Butter a 9-by-13-inch casserole dish. Heat the oil in a saucepan; add
the onions, garlic, and thyme, and cook over medium heat until the onions are translucent. Add
the cream and nutmeg, bring to a boil, and simmer for 3 to 4 minutes. Season with salt and pepper, and whisk to combine. Set aside to cool slightly.
Combine the potatoes and celery root in a large bowl with the cream mixture, and stir to coat. Stir together the cheeses in a separate bowl. Layer the potato and celery-root slices to overlap in the casserole dish, sprinkle with the cheese, and repeat, alternating layers. Pour remaining cream over the gratin, and finish the top layer with cheese. Cover with foil, and bake for about 45 minutes. Remove foil, and continue to bake for a further 30 minutes, until golden. Let cool slightly to allow the gratin to set before serving.