Heat a large skillet over medium high heat. Add ground beef, and brown for about 5 minutes, breaking it apart with a wooden spoon.
1 pound ground beef
Add onions, green peppers and mushrooms. Saute until veggies are soft, about 5-7 minutes.
1 medium onion, 1 green pepper, 1 cup mushrooms
In a small bowl mix together beef stock and cornstarch. Stir in with beef mixture. Add ketchup, Worcestershire sauce, salt and pepper. Reduce heat, and simmer for 5 to 10 minutes, until the sauce thickens.