To a large pan on the stove over medium heat add the sausage and onion break the sausage up and cook it through.
Bring a large pot of salted water to a boil on the stove then add in the pasta and cook it according to the package instructions drain and set to the side.
Remove any excess grease in the pan from the cooked sausage add in the pasta sauce, and water stir well. Stir in the cooked drained pasta.
Grease a 9×13 baking dish with non-stick spray. Add the casserole in. Sprinkle the mozzarella on top and bake for 35 minutes.