Preheat your barbecue to MEDIUM/HIGH. Pat the tuna steaks dry. Brush with oil and sprinkle with ground pepper, coarse salt and sesame seeds.
While the barbecue is heating, prepare the mayonnaise by mixing all the ingredients.
Sear the steaks on both sides so that they are golden brown on the outside but pink in the middle. Barbecue the buns for one minute. Top with mayonnaise, tuna and salad.