- 3 pounds celery roots (celeriac; about 4 medium), peeled, quartered, thinly sliced (about 7 cups)
- 3 pounds russet potatoes (about 4 large), peeled, thinly sliced (about 7 cups)
- 1/2 teaspoon ground nutmeg
- 1 large onion, thinly sliced
- 3 garlic cloves, minced
- 2 cups (packed) grated Gruyère cheese (about 8 ounces)
- 2 cups canned low-salt chicken broth or canned vegetable broth
- 1 cup whipping cream
- Preheat oven to 375°F. Butter 13 x 9 x 2-inch glass baking dish. Arrange 1/3 of celery root slices in dish. Top with 1/3 of potato slices. Sprinkle with salt and pepper, then 1/4 teaspoon nutmeg. Top with half of onion slices, half of garlic, then half of cheese. Repeat layering 1 more time. Cover with remaining celery root, then potatoes.
- Bring broth and cream to simmer in medium saucepan. Pour over vegetables. Sprinkle generously with salt and pepper. Cover baking dish with foil. Bake casserole 1 hour. Remove foil. Bake until vegetables are very tender and liquid bubbles thickly and is slightly absorbed, about 55 minutes longer. Remove from oven. Let stand 15 to 20 minutes; serve. (Can be made 2 hours ahead. Let stand at room temperature. Rewarm, covered with foil, in 350°F oven, about 25 minutes.)