In a large skillet, cook chopped onion and red pepper strips in hot cooking oil for 2 minutes. Add sliced mushrooms and stir-fry over medium-high heat for 2 to 3 minutes or until vegetables are tender.
Stir in balsamic vinegar, soy sauce, and Worcestershire sauce. Cook and stir for 1 to 2 mintues or until most of the liquid is absorbed.