Brown the ground beef over med-heat in a large skillet for about 5-6 minutes. Add the chopped onion, green pepper and mushrooms and cook another 3-4 minutes, until they start to get tender. Stir in the steak sauce and beef stock, season with salt and pepper, bring up to a boil and cook about 2 minutes.
Turn on your oven broiler.
Spoon a scoopful of the meat on the bottom half of the bun. Drizzle additional steak sauce on top. You can either place the cheese directly on the meat and place under the broiler to melt OR place one slice of cheese on bun top and toast under broiler. Put the sandwich together and serve with a tasty side. That's it. Seriously, these things are GOOD!