Root Vegetable Gratin

Name: Root-Vegetable Gratin
Ingredients: 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