Mix together onion, garlic, sugar, soy sauce, Chinese wine, mustard and sesame oil. Add beef steaks and mix well to coat. Cover and refrigerate for 2-3 hours to let the flavours infuse.
Drain steaks and cook on a preheated grill for:
Rare - 5 minutes,
Medium - 8 minutes,
Well done - 12 minutes.
Serve beef steaks with mixed vegetable and noodle stirfry. Add some of the remaining marinade, but ensure the sauce is thoroughly boiled prior to serving.