Melt butter in a large skillet over medium-low heat. Add onion; cook and stir until softened and translucent, about 5 minutes.

Add sausage, black pepper, salt, garlic powder, and pepper flakes; cook and stir until sausage is browned and crumbly, about 10 minutes.

Stir in flour until well blended; cook for about 2 minutes. Increase heat to medium. Slowly stir in milk; cook and stir until gravy thickens and is heated through, about 5 minutes.