Coat a large skillet with cooking spray, and place over medium heat. Add mushrooms and onions; cook and stir for about 4 minutes, just until mushrooms begin to give up their juices. Stir in bread cubes and steak sauce; cook and stir 1 minute. Remove mixture from pan, and set mixture aside to cool. Wipe pan clean.
Beat egg, and mix into mushroom mixture. Season to taste with salt and pepper.
Coat skillet with cooking spray, and place over medium heat. Form 6 patties by hand and place into skillet. Brown one side, flip over, and brown other side. Serve.