Cut the tuna into large chunks. Marinade in the olive oil, rosemary and garlic for an hour or two. Wrap each chunk of tuna in a strip of Parma ham and thread onto wooden skewers that have been soaked in warm water for 10 minutes. BBQ for a few minutes - the tuna should be very rare in the middle. If you have a lot of woody rosemary, it's good to use rosemary sticks in place of the skewers.