1-2 long sweet potatoes (about 2 inches thick), peeled
4 large parsnips, ends trimmed and peeled
5 medium beets, peeled
2 1/4 cups heavy cream, divided (whole milk is fine, but mixture wont thicken as much or be as creamy)
4 ounces grated Parmesan, divided
1 tablespoon fresh minced thyme, divided, plus more for garnish
1 garlic clove, minced
2 ounces shredded gruyere
salt and pepper to taste
1. Preheat oven to 400˚F.
2. Slice sweet potato, parsnips and beets into very thin rounds (using a mandoline is easiest) and transfer each vegetable to its own bowl.
3. Pour 2/3 cup cream over each bowl of vegetables and top each with 1/2 ounce grated Parmesan and 1 teaspoon minced thyme. Season each bowl generously with salt and pepper and toss together until all vegetable slices are well coated.
4. Pour remaining cream into the bottom of a 2 quart (oval) baking dish and sprinkle with 1/2 ounce of Parmesan and minced garlic.
5. Grab a stack of sweet potatoes and line them standing up on a bias, at the top of the baking dish.
6. Follow the sweet potato with a row of parsnips, followed by a row of beets.
7. Repeat with the remaining sweet potatoes, parsnips and beets, creating 6 rows of root vegetables.
8. Season top of gratin with salt, pepper and sprinkle of remaining ounce of Parmesan.
9. Cover with foil and bake for 25 to 30 minutes or until vegetables are soft.
10. Remove from oven, uncover gratin and top with shredded gruyere.
11. Set oven to broil and broil gratin for 3 to 5 minutes or until browned and bubbling on top.
12. Finish with a sprinkle of fresh thyme leaves. Serve.