Ingredients

• 1 lb parsnips (about 4 medium)
• 1 medium celery root (sometimes called celeriac; 3/4 lb total)
• 1 lb sweet potatoes
• 1 1/2 lb russet (baking) potatoes
• 2 teaspoons salt
• 1 teaspoon finely chopped garlic
• 1/2 teaspoon black pepper
• 1/4 teaspoon freshly grated nutmeg
• 1/2 cup reduced-sodium chicken broth
• 1 3/4 cups plus 2 tablespoons heavy cream
• Special equipment: an adjustable-blade slicer; a 3-quart gratin or other shallow flameproof baking dish (not glass; 13 by 9 inches)

Instructions

1. Put oven rack in upper third of oven and preheat oven to 400°F.
2. Peel parsnips and cut crosswise into 3-inch lengths with a knife, then cut lengthwise around core into 1/8-inch-thick slices with slicer (discard core). Transfer to a large bowl.
3. Peel celery root and all potatoes (prepare russet potatoes last to avoid discoloration) and halve lengthwise, then cut crosswise into 1/8-inch-thick slices with slicer. Add to parsnips along with salt, garlic, pepper, nutmeg, broth, and 1 3/4 cups cream, tossing to combine. Transfer to gratin dish, spreading evenly.
4. Cut out a piece of parchment or wax paper to fit just inside gratin dish, then butter 1 side of parchment. Cover vegetables directly with parchment, buttered side down, then put dish in a shallow baking pan (to catch any drips). Bake until gratin is bubbling all over and vegetables are tender when pierced with a knife, about 50 minutes. Discard parchment.
5. Just before serving, preheat broiler. Drizzle top of gratin with remaining 2 tablespoons cream and broil 4 to 6 inches from heat until browned, 2 to 3 minutes.