How to Make It
Heat oven to 425°F. Cut the sweet potatoes crosswise into ¼-inch slices, leaving about ⅛ intact at the bottom. Place the sweet potatoes in a baking dish. Stir the thyme into the melted butter, then brush onto the sweet potatoes so it seeps in between the slices. Bake until golden and tender, 50-55 minutes.
Turn oven to broil. Combine the Gruyere and Parmesan in a small bowl. Divide the cheese mixture evenly among the potatoes, stuffing the cheese in between the slices. Broil until the cheese is melted and golden brown, 2 to 3 minutes. Season with salt and pepper.