Cook sausage in a cast-iron skillet. When done, remove sausage from pan and pour off all but 2 tablespoons of fat. Whisk flour into the fat and cook over low heat for 5 minutes. Remove pan from heat and whisk in milk a little at a time. Return to medium-high heat and stir occasionally while the gravy comes to a simmer and thickens. (Be sure to scrape up any brown bits that might be stuck to the bottom of the pan — that's where the flavor is.) Check seasoning, adding salt and pepper as desired, add crumbled sausage, and serve over toast or biscuits.