Remove sausage from casing, break up to small pieces
Add sausage to a cold pan. slowly bring up heat to allow fat from sausage to render. Cook sausage though completely and remove from pan, leaving any excess fat in pan.
Add butter to pan to make 4 total tablespoons of fat. Melt butter. Add flour and whisk to combine.
Cook roux until raw flour taste is cooked out and begins to smell nutty.
Add about 1/2 cup of milk and whisk until totally combined.
Repeat with about 1 cup milk. Whisk until fully incorporated.
Add the remaining milk. Keep at a simmer while whisking constantly until desired thickness is achieved. Keep in mind that the gravy will thicken as it cools.
Season with salt and pepper to taste. Add sausage back to gravy. Mix to combine and let cool slightly before serving.