Balsamic Steak Tips

In a bowl, combine the beef, balsamic vinegar, soy sauce, garlic, Dijon mustard, olive oil, and red pepper flakes. Mix well and marinate for 4 hours. In a pan, cook the onion and mushrooms, seasoning with salt and pepper, until golden. Remove the cooked onion and mushrooms from the pan. Add the marinated beef with the sauce to the pan and cook for 10 minutes. Add back the cooked onions and mushrooms to the pan with the beef and cook until tender (approximately 6 more minutes). Serve over mashed potatoes.