Place onion, garlic, olive oil, vinegar, soy sauce, rosemary, mustard, salt, and pepper into the bowl of a food processor. Process until marinade is smooth.

Place steak in a large resealable plastic bag. Pour marinade over steak; seal the bag and refrigerate for about 3 hours.

Preheat the grill for high heat. Brush the grate with oil.

Remove steak from marinade and place on the prepared grill grate; discard marinade. Cook on the preheated grill to desired doneness or about 7 minutes per side.