Brown sausage in a pan, breaking up and crumbling.
Once the meat is fully browned, scoop it out with a slotted spoon, leaving any drippings in the skillet.
Add the butter or bacon fat to the same skillet and bring the pan up to medium heat.
Slowly whisk in the flour, stirring constantly until smooth, cooking for at least 5 minutes.
Slowly begin whisking in the first two cups of milk until fully incorporated and mixture begins to bubble. This will provide thick gravy. Continue whisking in additional milk a little bit at a time, until the gravy reaches the desired consistency.
Grind some pepper directly into the gravy, add salt to taste, and return the sausage to the skillet.