With a mandolin slice the cucumber lengthwise to create thin long strips.
In a bowl combine tuna, black sesame seeds, sesame oil, and freshly ground pepper. Reserve.
Lay one strip of the cucumber flat on a clean surface.
Place a spoon of the tuna mixture at one end and roll up like a sushi roll. Place a touch of wasabi paste at the other end to seal.
Slice roll in half, displaying the green part of the cucumber on the top.
In a small bowl, mix together mayonnaise and the chili paste. Top each cucumber roll with a small dollop.