Sweet Potato, Potato, and Turnip Gratin with Thyme and Gruyere

Sweet Potato, Potato, and Turnip Gratin with Thyme and Gruyere
 
1 pound of potatoes
1 pound medium red-skinned sweet potatoes
1 pound of turnips
2 cups heavy whipping cream
1/4 cup (1/2 stick) butter
2 garlic cloves, minced
2 tablespoon minced fresh thyme
1 1/2 teaspoons fine sea salt
3/4 teaspoon freshly ground black pepper
1 1/4 cups (packed) coarsely grated Gruyère cheese (about 5 ounces)
 
Using a mandoline (or knife if you do not have a mandolin), cut the potatoes, sweet potatoes, and turnips in 1/8-inch rounds.  Combine cream, butter, and garlic in medium saucepan; bring to simmer. Remove from heat. Place thyme leaves in a small bowl. Mix sea salt and black pepper in another small bowl.

Butter small baking dish or pie pan. Alternate slices of sweet potato, potato, and turnip, standing on their skin side, to fill the dish or pan.  Sprinkle thyme and salt mixture over the slices, separating if necessary and allowing some of the salt and herbs to fall between the slices.  Pour cream mixture over gratin, pressing lightly to submerge potato mixture as much as possible. Sprinkle the cheese on top. 
DO AHEAD Can be made 6 hours ahead. Cover with plastic wrap and chill. Remove plastic wrap before baking.
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Preheat oven to 400°F. Cover gratin tightly with foil. Bake 30 minutes. Uncover; bake until top of gratin is golden and most of liquid is absorbed, about 25 minutes longer. Let stand 10 minutes; serve.

With a salad, it makes a meal!