In a large skillet, brown sausage until no pink is left. Remove from pan but keep the drippings
Melt the butter in the skillet with the drippings and add the flour. Combine until smooth and cook for a couple of minutes
Whisk in the milk and cook until thickened and it coats the back of a spoon, about 2 minutes
Return sausage to pan, add salt and pepper, and cook for 10 more minutes or until desired thickness. You might need to add a little more milk if necessary.