Preheat your oven to 400°F (200°C) and grease a 3-quart oval baking dish with softened unsalted butter.

Slice sweet potatoes, parsnips, and beets into thin rounds and place each in separate bowls.

In the bowl with sweet potatoes and parsnips, add 4 tablespoons heavy cream, ½ ounce grated Parmesan, and 1 teaspoon minced thyme. For the beets, add 2 tablespoons of cream and the same amount of Parmesan and thyme, and season with salt and pepper.

Pour ¼ cup of cream into the bottom of the greased baking dish, sprinkle with ½ ounce of Parmesan cheese and minced garlic.

Layer sweet potatoes, parsnips, and beets diagonally in the baking dish, seasoning with salt, pepper, and remaining Parmesan on top.

Cover the dish with aluminum foil and bake for 30 minutes.

Remove the foil and add shredded gruyère cheese on top, bake uncovered for an additional 18–20 minutes or until golden brown.

Let it cool for 10 minutes before serving and garnish with fresh thyme leaves.