Remove sausage casings and crumble into a 10-12" frying pan over medium heat. Add onion and cook, stirring frequently. until sausage is well browned, about 10 minutes. Discard fat, sand s sauce, mushrooms. Heat until bubbly, then pour into an ungreased 9x13" baking dish. Spread mozzarella even over the top.
In blender whirl eggs, milk, oil, flour and salt until smooth. Pour evenly over casserole, sprinkle with parmesan cheese. Bake uncovered in a 400 degree oven until crust is puffed and golden brown, about 25 minutes. You might wait until after baking to sprinkle cheese as crust puffs better that way.