3/4 teaspoon each salt and pepper, or to taste (divided)
Pinch freshly grated nutmeg
2 medium celery roots, about 1 pound each, trimmed, peeled and sliced paper-thin (using a mandoline)
1/3 cup freshly grated Parmesan cheese
2 tablespoons minced fresh thyme
1 cup shredded Gruyère cheese
Melt butter in skillet over medium heat; sauté shallots and garlic until slightly translucent, about 1 minute. Add cream, season with some of the salt and pepper and nutmeg; bring to simmer. Remove from heat and keep warm.
Generously grease a three-litre shallow baking or gratin dish. Layer about half of the celery root slices in the dish, overlapping slightly. Season with some of the remaining salt and pepper. Stir the cream mixture and pour half over the celery root. Sprinkle all of the Parmesan and half of the thyme over top. Top with remaining celery root, cream mixture and thyme. Cover the dish with aluminum foil.
Bake in 350 degrees Fahrenheit (180 degrees Celsius) oven until tender when pierced with a knife, about 1 hour. Uncover and sprinkle with Gruyere; broil, just until cheese is melted and lightly browned.
Let stand 15 to 20 minutes before serving to allow the liquid to absorb back into the vegetables.