Cook the Sausage: Heat a skillet over medium heat and cook the sausage, breaking it up into crumbles, until browned. Remove the sausage and set it aside.
Make the Roux: Add butter to the skillet with the sausage drippings. Once melted, whisk in the flour and cook for 1-2 minutes.
Add the Milk: Slowly whisk in the milk, about ½ cup at a time, until smooth and thickened (5-7 minutes).
Season the Gravy: Stir in the black pepper, sage, red pepper flakes (if using), and a pinch of salt. Adjust seasoning to taste.
Combine with Sausage: Return the cooked sausage to the skillet, stirring it into the gravy. Simmer for 2-3 minutes.
Serve: Split warm biscuits in half and spoon the sausage gravy generously over the top. Enjoy!