1. Heat oven to 400°. Combine cheeses in a large mixing bowl and set ⅓ aside. Add cream, garlic, thyme, nutmeg, and salt to remaining cheese mixture and whisk together. Add potatoes and use your hands to separate the slices and make sure each one is coated well. Season with pepper.
2. Grease a 9-inch cast iron skillet or baking dish with butter. Arrange sweet potato slices vertically (standing on their side) filling the pan. Pour remaining cream mixture over potatoes.
3. Cover with foil and bake for 30 minutes. Uncover and bake for another 30 minutes. Sprinkle remaining cheese over potatoes and bake until golden brown, about 10 minutes.