- 1 1/2 pounds medium Yukon Gold potatoes
- 1 1/2 pounds medium red-skinned sweet potatoes (yams)
- 2 cups heavy whipping cream
- 1/4 cup (1/2 stick) butter
- 2 garlic cloves, minced
- 1 tablespoon minced fresh Italian parsley
- 1 tablespoon minced fresh rosemary
- 1 tablespoon minced fresh sage
- 1 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 Gruyére cheese (about 5 ounces)
- Fill large bowl with cold water. Working with 1 Yukon Gold potato at a time, peel, then cut into 1/8-inch-thick rounds and place in bowl with water. Repeat with sweet potatoes.
- Combine cream, butter, and garlic in medium saucepan; bring to simmer. Remove from heat. Mix all herbs in small bowl. Mix sea salt and black pepper in another small bowl.
- Butter 13x9x2-inch glass baking dish. Drain potatoes, then pat dry with kitchen towels. Transfer half of potatoes to prepared baking dish. Use hands to distribute and spread evenly.
- Sprinkle with half of salt-pepper mixture, then half of herb mixture. Sprinkle with half of cheese. Repeat with remaining potatoes, salt-pepper mixture, herb mixture, and cheese.
- Pour cream mixture over gratin, pressing lightly to submerge potato mixture as much as possible. This can be made 6 hours ahead. Cover with plastic wrap and chill. Remove plastic wrap before baking.
- 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.
- Side Dishes, Potato