Preheat oven to 350°F.

In a large skillet over medium heat, melt the butter. Add the sweet potato and onion, season with salt and pepper, and cook for 10-12 minutes.

Beat the eggs in a medium mixing bowl, then add the sour cream and beat again until combined.

Spread the vegetables out evenly in the skillet and pour the egg mixture on top. Sprinkle with cheese.

Transfer skillet to oven and cook for 12-15 minutes. Let the frittata rest in the skillet for at least 5 minutes, then slice and serve.