We love the earthy, sweet flavor of the root vegetable combination in this cheesy gratin, but you could also go with the classic all-potato version (use 5 large potatoes, about 3 1/2 lbs. total, in place of the celery root and parsnips).
2 cups heavy cream
1 teaspoon kosher salt
1/2 teaspoon freshly ground nutmeg
3 large russet potatoes (about 2 lbs.), peeled and thinly sliced
2 medium parsnips (about 10 oz.), peeled, trimmed, and thinly sliced
1 large shallot, thinly sliced
1 medium celery root (about 15 oz.), peeled, halved, and thinly sliced
Preheat oven to 325°. In a small saucepan, bring cream, salt, and nutmeg to a simmer. Remove from heat and set aside.
Lay half the potatoes evenly in a shallow 2 1/2-qt. baking dish, followed by half the parsnips and all the shallot and celery root. Top with half the cheese. Add remaining parsnips, then remaining potatoes. Pour warm cream over vegetables. Sprinkle evenly with remaining cheese.
Bake until bubbling, golden brown, and tender when pierced, about 1 1/4 hours. Let sit 10 minutes before serving.
Make ahead: Up to 2 days; reheat at 325° until center is warm, about 30 minutes.