Crumble the sausage in a 10- to 12-inch cast iron skillet. Brown over medium heat. Remove the cooked sausage and pour off all but 2 tablespoons of fat, making sure you leave the stuck-on bits behind.
Whisk the flour into the fat in the skillet and cook over medium heat for 3 minutes. Kill the heat and whisk in the milk a little at a time. Return the mixture to medium-high heat and whisk occasionally while the gravy comes to a simmer and thickens, 2 to 3 minutes. (Be sure to scrape up any brown bits that might be stuck to the bottom of the pan — that's where the flavor is.)
Crumble the cooked sausage into the gravy, season with salt, add an insane amount of pepper, and serve hot over toast or biscuits.