Place a cast iron skillet over medium-high heat and add the breakfast sausage. Break it into small crumbles as it cooks. Allow the sausage to brown well so it develops a deep, savory flavor.
Stir the diced onion into the browned sausage. Cook until the onions become soft and lightly golden. This step adds sweetness and creates a more balanced gravy.
The sausage should release enough fat to form the base of the gravy. If there isn't enough rendered fat in the skillet, add a small amount of butter. The fat is necessary for creating a smooth roux.
Sprinkle the flour evenly over the sausage mixture. Stir continuously until all dry flour disappears. Cooking the flour briefly removes its raw taste and improves the texture.
Pour in a small amount of milk first and stir until absorbed. Slowly add the remaining milk while whisking constantly. This prevents lumps from forming and creates a silky consistency.
Bring the mixture to a gentle simmer. Continue stirring while the gravy thickens and coats the back of a spoon. This usually takes a few minutes.
Add salt and freshly ground black pepper to taste. The pepper is an important flavor component and gives the gravy its classic Southern character.
While the gravy cooks, warm your biscuits in a low-temperature oven. Warm biscuits help the gravy stay hot longer and improve the overall eating experience.
Split the biscuits and place them on serving plates. Generously spoon the hot sausage gravy over the top. Serve immediately while everything is warm and fresh.