Preheat oven at 375. Spray 1 1/2 quart baking dish with cooking spray. Brown sausage in skillet with onion. Drain off fat. Mix the rest of the ingredients together in a bowl except for the shredded cheese. Add 1/3 c. of shredded cheese. Then add meat mixture. Mix well. Spread evenly into baking dish. Bake for 45-50 minutes or until casserole is brown and the middle is set. During last 15 minutes sprinkle remain 1/3 cup shredded cheese over the top and return back to oven. Let stand for 10 minutes before cutting.