Preheat oven to 400 degrees F. Spray a 3-quart baking dish with cooking spray and set aside.
Crumble the sausage in a skillet and cook over medium-high heat until no longer pink (about 5-7 minutes). Use a wooden spoon to break the sausage into small pieces as it cooks. Use a slotted spoon to remove the cooked sausage to a paper-towel lined plate. Leave no more than 1 tablespoon of fat drippings in the pan (discard any excess).
Add the butter to the skillet with the drippings. Melt butter over medium heat. Sprinkle flour over the melted butter and stir to combine (it will form a thick paste). Slowly whisk in the milk, sage, pepper and salt; bring to a bubble, whisking until smooth and thick (about 3 minutes). Stir the cooked sausage back into the gravy.
Transfer the gravy to the prepared baking dish. Place biscuit quarters in an even layer on top of the gravy.
Bake casserole (uncovered) until the biscuit topping is golden brown and cooked through (about 15-20 minutes). Let the casserole stand for 5 minutes before serving.
Notes
Swap out the pork sausage for an equal amount of ground beef, or use a different type of sausage (such as turkey sausage or Italian sausage).
Can you make this biscuits and gravy casserole the night before? Yes. To do so, prepare the gravy the day before and place it in the prepared baking dish. Cover and refrigerate. In the morning, allow the gravy to sit on the counter for at least 30 minutes so that it comes to room temperature. Add the biscuit dough on top, and bake according to the recipe instructions.
How long will biscuits and gravy casserole keep in the refrigerator? This recipe will last in the fridge for 3-4 days.
How to reheat the casserole: Cover the casserole loosely with foil. Heat in a 325 degree F oven for about 20 minutes, or until heated through.
How to freeze biscuits and gravy casserole: You can freeze this casserole after baking. Allow the dish to cool to room temperature. Cover tightly and freeze for up to 3 months. Before reheating, thaw in the refrigerator overnight.