In a large, heavy-bottomed skillet, melt the butter over medium high. Add the sausage and brown, using a wooden spoon to break up the meat into small pieces. Continue to cook, stirring every minute or so, until it is cooked through and no longer pink, 6 to 8 minutes. With a potato masher, break the sausage up into super fine crumbles. Stir in the black pepper, sage, and salt. If at any point the meat starts to stick (this won't happen with pork but might with turkey), add a little bit of oil or additional butter.

Reduce the heat to medium. Sprinkle the flour over the top. Stir to coat the sausage with the flour. Continue cook, stirring constantly, for 2 full minutes. No raw bits of flour should remain.

Slowly pour in the milk, stirring constantly to prevent lumps. Increase the heat to bring to a gentle simmer. Let bubble and reduce, constantly running a wooden spoon along the bottom to prevent sticking, until the gravy is thick enough to coat the back of a spoon, 5 to 7 minutes. Taste and adjust seasoning as desired (I kept a sage on this fairly subtle, but feel free to add more if you like a stronger herby flavor). Serve hot over the biscuits.