Preheat the oven to 350°F (176°C). Coat a 9-inch (23cm) baking dish with nonstick spray.
Arrange the sweet potato slices in a circular pattern in the baking dish. Season with salt and pepper. Transfer to the oven to bake for 1 hour.
During the last few minutes of baking, combine the butter, garlic and sage in a small pan and cook on medium heat until the sage is crisp, and the butter is slightly golden brown.
Remove the sweet potato from oven and place on a trivet. Drizzle the sage butter sauce evenly as possible over the sweet potatoes. Sprinkle on the cheese, and serve.
Notes
Parmigiano reggiano or any type of hard, salty cheese is a good substitute for pecorino romano.