"Flank steak is a perfect grilling meat. It's marbleized with fat that melts while grilling and fills the meat with the flavor of natural juices. If you have a lot of marinade left over from this recipe, save it in your refrigerator and use it to marinate other grilled meats or chicken; it will last up to one week."
Combine onion, garlic, oil, vinegar, soy, mustard, rosemary, salt and pepper in a mixing bowl using a wire whisk or blender until well combined, about 30 seconds.
Place meat in large zip lock plastic bag. Pour marinade over meat. Refrigerate steak for a minimum of 30 minutes or up to 2 days.
Place marinated flank steak on grill over hot coals. Brush with extra marinade throughout grilling. Grill until golden and cooked to desired doneness, about 6-8 minutes on each side (medium-rare). Transfer grilled flank steak to a serving plate or dish to collect juices.
Using a sharp knife, slice the meat thinly across the grain.
Serve slices with desired accompaniments; drizzle with collected meat juices.