Place onion, garlic, olive oil, vinegar, soy sauce, rosemary, mustard, salt, and pepper into the bowl of a food processor. Process until smooth. Place steak in a large resealable plastic bag. Pour marinade over steaks, seal, and refrigerate for about 3 hours.
Preheat the grill for high heat.
Brush grill grate with oil. Discard marinade, and place steak on the prepared grill. Cook for 7 minutes per side, or to desired doneness.
