1. In a medium bowl, combine oil, soy sauce, Worcestershire sauce, vinegar, lemon juice, mustard, pepper, garlic, and onion. Mix together well.
2. Rinse beef with cold water and pat dry. Cut the beef into desired portions. Put the steaks in a resealable plastic bag, pour in the marinade, squeeze out excess air, seal the bag and turn to coat. Let stand at room temperature for about 1-2 hours.
3. Remove the steak from the marinade, shake off excess, and discard the remaining marinade.
4. Heat 2 tablespoons oil over medium-high heat until really hot. Add the steaks and cook about 7 minutes per side for medium, or till the desired degree of doneness is achieved.
5. Allow to rest for 5 minutes before serving.