- 1 tablespoon unsalted butter (softened)
- 1-2 l ong sweet potatoes (about 2 inches thick) peeled
- 3-4 large parsnips ends trimmed and peeled
- 3-5 small beets peeled
- 14 tablespoons heavy cream divided ((whole milk is fine, but mixture won't thicken as much or be as creamy))
- 4 ounces grated Parmesan divided
- 1 tablespoon fresh minced thyme divided (plus more for garnish)
- 1 g arlic clove minced
- 1 ounce shredded gruyere
- salt and pepper to taste
Mandoline slicer
3-quart baking dish
Aluminum foil
Cheese grater
Prepare the Oven and Baking DishPreheat the oven to 400°F (200°C). Generously butter a 3-quart baking dish, making sure the bottom and sides are well coated to prevent sticking. Slice the VegetablesUsing a sharp knife or mandoline, slice the sweet potatoes, parsnips, and beets into very thin, evenly sized rounds. Keeping the slices similar in thickness helps the vegetables cook at the same rate.Place each vegetable in a separate bowl. If using fresh beets, consider wearing gloves while handling them to prevent their natural color from staining your hands. Season the VegetablesPour 4 tablespoons of cream over the sweet potatoes and parsnips. Pour 2 tablespoons of cream over the beets.Add Parmesan cheese, thyme, salt, and black pepper to each bowl. Gently toss the vegetables until the slices are evenly coated with the cream and seasonings. Prepare the Creamy BasePour the remaining ¼ cup of cream into the bottom of the prepared baking dish. Sprinkle the cream evenly with Parmesan cheese and minced garlic. Arrange the Vegetable SlicesBegin arranging the vegetables in the baking dish. Stand a stack of sweet potato slices upright at a slight angle, then add slices of parsnip followed by beet slices.Continue alternating the vegetables in the same pattern until the entire dish is filled. Keep the slices standing closely together so the gratin holds its shape while baking. Bake CoveredSeason the arranged vegetables with additional salt and black pepper, then sprinkle more Parmesan over the top.Cover the baking dish tightly with aluminum foil. Bake for 30 minutes, allowing the vegetables to steam and begin softening in the creamy mixture. Add the Gruyère and Finish BakingCarefully remove the foil and sprinkle the shredded Gruyère evenly over the top.Return the uncovered dish to the oven and bake for another 18–20 minutes, or until the vegetables are fork-tender and the Gruyère is fully melted, bubbling, and lightly browned. Rest and ServeRemove the gratin from the oven and let it rest for a few minutes before serving. This allows the creamy sauce to settle and makes the gratin easier to portion.
Recipe Notes
Yield: Makes 1 3-quart casserole.
Nutrition: Nutrition information is based on the lowest quantity of vegetables listed in the ingredient ranges.
Make Ahead: The gratin can be prepared up to 1 day ahead. Cover tightly with foil and refrigerate. When ready to bake, place the covered gratin directly in the oven and add about 5–7 minutes to the covered baking time.
Beet Color: Preparing the gratin ahead can cause the beets to bleed their color into the cream. This does not affect the flavor, but it may change the appearance of the finished dish.
Prevent Beet Bleeding: Coat the sliced beets only lightly with cream. They should be covered enough to prevent them from drying out during baking, but avoid using excess cream.
Creamy Base: The bottom of the baking dish should have only a thin layer of cream topped with Parmesan. The exact amount may vary slightly depending on the surface area of your baking dish, so use just enough to lightly coat the bottom.
