Step 1Generously butter a 2-quart gratin dish and set aside. Heat the oven to 425 degrees.
Step 2Peel the sweet potatoes. Using a mandoline or sharp knife, cut into slices one-fourth-inch thick. Arrange the slices in overlapping layers in the dish, seasoning each layer generously with salt and pepper.
Step 3Combine the cream and thyme in a small bowl and mix well. Pour evenly over the potatoes, coating all exposed surfaces. Cover the dish tightly with foil and bake 45 minutes.
Step 4Remove the foil; continue baking until the sweet potatoes are soft and caramelized, 15 to 20 minutes longer.