If using onion, sauté in a small amount of vegetable oil or bacon fat in a deep skillet until translucent.
Add sausage & brown, breaking up into small bits. When browned, sprinkle flour over & stir until flour is browned. If necessary, add extra fat or oil - approx. 2-4 Tbsp - before adding flour.
There should be approximately 4-5 Tbsp fat. This will be a fairly dry roux so be careful not to let it scorch. When flour has colored slightly add cold milk.
Continue cooking gravy over med-high heat until it starts to thicken. Reduce heat to low & continue cooking, stirring frequently, until gravy is very thick.
Servings: