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, add crumbled sausage and serve over toast or biscuits.