Heat a large cast iron skillet and add the sausage. Break up into small pieces as it cooks.
Add the onion and saute until caramelized.
Sprinkle flour on top and cook until flour, sausage, and onions form a roux. Add pepper to taste.
Heat milk in microwave for 5 minutes. Add to the skillet slowly, stirring constantly. Bring to a boil while continuing to stir.
Bake biscuits until brown and crispy on the outside, soft on the inside.
Allow gravy to boil for one minute, then reduce heat to medium/low. Continue to stir, scraping the bottom of the pan. while waiting for gravy to thicken. This may take 15-20 minutes.
Serve gravy over split biscuits.