Lay out a sushi nori sheet, shiny side down. Into the middle 1/3 of the sushi nori sheet place a lettuce leaf that goes from edge to edge, then onto the top of this spoon on the tuna, add the mayonnaise to the top of the tuna and then the sliced cucumbers, peppers and green onion.
Wet the top edge of the sushi nori (furthest away from you) using your finger, then roll to make a seal, cut in half and serve.