- Nonstick cooking spray
- Coarse salt and freshly ground pepper
- 3 cups grated Parmesan cheese
- 1 1/4 pounds sweet potatoes, peeled and cut into 1/8-inch-thick slices
- 1 1/4 pounds yellow beets, peeled and cut into 1/8-inch-thick slices
- 1 1/4 pounds parsnips, peeled and cut into 1/8-inch-thick slices
- 1 1/4 pounds celery root, peeled and cut into 1/8-inch-thick slices
- 1 1/4 pounds Yukon gold potatoes, peeled and cut into 1/8-inch-thick slices
- 2 cups heavy cream
Preheat oven to 350 degrees.
Spray a shallow 9-by-13-inch baking dish with nonstick cooking spray. Sprinkle bottom of dish with salt and pepper. Set aside 1 cup cheese. Place a layer of sweet potatoes, in an overlapping fashion, on bottom of baking dish; sprinkle with some of the remaining 2 cups cheese. Top with a layer of yellow beets, overlapping in the same fashion; sprinkle with more of the remaining cheese. Repeat process making a layer of parsnips, followed by celery root, and Yukon gold potatoes, topping each layer evenly with some of the remaining cheese.
Gently pour cream over vegetables. Cover baking dish with parchment paper-lined aluminum foil; place baking dish on a rimmed baking sheet. Transfer to oven and bake until a knife is easily inserted into gratin. Uncover and sprinkle reserved cheese over top; increase oven temperature to 425 degrees. Return baking dish to oven and continue baking until cheese is browned and bubbly, about 15 minutes more.
Remove baking dish from oven and let gratin stand 30 minutes before serving.