Melt butter in a skillet over medium-low heat; stir in onion. Cook and stir until the onion has softened and turned translucent, about 5 minutes.
Add sausage, pepper, salt, garlic powder, and red pepper flakes. Cook and stir until sausage is browned and crumbly, about 10 minutes.
Stir in flour until well blended and cook for about 2 minutes. Increase heat to medium and slowly add milk, stirring as you go. Cook and stir until gravy thickens and is heated through, about 5 minutes.
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