Mixed Potato Gratin

INGREDIENTS

• 3 large Russet potatoes
• 3 large red-skinned sweet potatoes
• 1 1/2 cups heavy cream
• 2 garlic cloves, minced
• 1 tablespoon minced fresh thyme
• 1 1/2 teaspoons fine sea salt
• 1/2 teaspoon freshly ground black pepper
• 1 1/2 cups shredded Gruyère cheese
• 3/4 cup shredded parmesan cheese
• 2 tablespoons butter, cut into small pieces