Drain the oil from the can of tuna and place the tuna in a mixing bowl. Add the mayonnaise, minced onion, rice vinegar, wasabi paste, salt, and black pepper. Mix thoroughly until the ingredients are well combined into a creamy salad.

Cut six 1-inch squares from the half sheet of sushi nori using scissors. Then, cut each square diagonally in half to make two triangular ‘sails’ and set these aside for garnish. Cut the remaining nori into long, thin strips and then into small squares. Stir these small nori squares into the tuna salad from Step 1 for added umami.

Trim the ends off the English cucumber and cut it into six 2-inch sections. Slice each section lengthwise in half to create two 'boats' per section. Stand each cucumber boat on end and trim a thin slice of peel from the rounded side so the boat sits flat, cut side up. Then, using a melon baller or small spoon, scoop out the seeds from each cucumber boat, leaving about a 1/4-inch wall at each end. I find using a melon baller makes this step quick and neat.

Fill each hollowed cucumber boat with about one tablespoon of the wasabi tuna salad (from Step 2), mounding the filling slightly. Use a butter knife to make a shallow slit in the tuna salad, then insert a nori triangular sail into the slit. Gently pinch the tuna salad around the base to hold the sail upright. Arrange the finished tuna boats on a serving platter.

If using, cut a handful of cherry tomatoes in half and scatter among the tuna boats. Sprinkle the sesame seeds over the boats and tomatoes. Serve immediately for best flavor and texture, or refrigerate the tuna salad and cucumber boats separately and assemble just before serving. To make the dish even more delicious, I like to use freshly toasted sesame seeds for garnish—they add a wonderful nutty aroma.