Prepare the Macaroni and Cheese as directed on the box, including the sauce mix.
Preheat oven to 350 degrees F.
In a large skillet brown the beef. Add the chopped onion in the last few minutes and cook until onion is clear. Drain any fat off of the meat.
In the prepared mac & cheese, fold in 1/4 cup cheddar cheese, mushroom soup, milk, and Worcestershire sauce, and pepper to taste. Add ground meat mixture and drained mushrooms. Mix well.
Spray a 8 x 8 casserole dish with nonstick cooking spray. Transfer mix into casserole and top with the remaining 1/2 cup cheddar cheese.
Bake for 30 minutes.