1. Trim boneless wagyu steaks of any visible fat. Place steaks in a large shallow dish; pour over combined olive oil, garlic, mustard, plum sauce, vinegar, rosemary and pepper. Turn steaks to evenly coat in marinade mixture. Cover; refrigerate for about 4 hours, or overnight.
2. Drain wagyu steaks from marinade; reserve marinade.
3. Cook wagyu steaks, on an oiled, heated barbecue, for about 3 minutes on each side, brushing reserved marinade over steaks using extra rosemary sprigs, or until cooked as desired.
4. Serve steaks with barbecued baby potatoes, a green salad or a variety of in season vegetables. Yields 12.