In a 10-inch cast iron skillet over medium-high heat, cook the sausage, breaking it apart with a wooden spoon, until it has released at least ¼ cup of fat, which should take about 6 minutes. If the sausage is lean and doesn't yield enough fat, add butter, one half tablespoon at a time, until you reach the desired amount.
Add the chopped onion and a pinch of salt to the skillet. Cook, stirring occasionally, until the onion is very soft and golden, and the sausage is nicely browned, which should take about 7 more minutes.
Mix in the flour until there are no dry bits left, which should take about 30 seconds.
Add 2 tablespoons of milk and stir until it is fully absorbed.
Gradually pour in the remaining milk while stirring continuously to prevent lumps from forming.
Bring the mixture to a simmer and cook, stirring and scraping the sides and bottom of the skillet, until a thick, silky gravy forms, which should take about 4 minutes.
Season the gravy to taste with salt and a generous amount of black pepper.
Serve the warm biscuits topped with the hot gravy.
