In a medium container, mix Worcestershire sauce and mustard. Place steak strips in the mixture. Cover, and marinate in the refrigerator at least 30 minutes.
Heat olive oil in a medium saucepan over medium heat. Stir in the onion and garlic, and cook until tender. Season with pepper. Place steak into the saucepan and cook 5 to 7 minutes on each side, to desired doneness. Discard remaining marinade.
To Freeze: Freeze in freezer safe container.
To Reheat: Reheat in a covered pan or, if packaged in a food saver bag, bring a pot of water to a simmer. Drop bag of food in and simmer until heated through.