In a large skillet, brown the sausage and onions. Pop the buttermilk biscuits into the oven while the sausage is browning. Remove the sausage from the pan, and reserve about three tablespoons of grease. Stir about three tablespoons of flour in with the grease. (Always keep about equal measurements for grease and flour.)
When you add the flour to the grease, you must stir constantly to prevent the flour from burning. Once the flour has browned nicely, pour in about one cup of regular milk. Season with salt and pepper, and add a sprinkling of parsley flakes. Return the sausage to the skillet, and cook on low until heated through. Serve hot.
Buttermilk biscuits go exceedingly well with this gravy, but you don't have to save this gravy for biscuits or even for breakfast. Sausage gravy can be served for any meal.