Prep. If you are making the biscuits fresh, don't start cooking the gravy until the biscuits are almost done. If you are using pre-cooked biscuits, warm them as you start the gravy.
Combine the flour, sage, fennel, chipotle, and pepper in a small bowl.
If the sausage is in links, remove the casings and crumble it.
Cook. Cook the crumbled sausage and onion in the bacon fat or butter in a large frying pan over medium heat until brown and warmed through.
Sprinkle the flour mixture over the sausage and onions and cook, stirring steadily, for about 2 minutes or until flour has soaked up the fat and gotten a bit tan in color.
Gradually stir in the milk and simmer for about 5 minutes until it thickens. If the gravy gets too thick, add more milk.
Serve. Split the biscuits open with a fork and spoon the gravy over the top. Wear a bib.