Cook the rice

• Rinse the sushi rice and cook it with the water until tender and the water is absorbed.
• Let the rice steam covered off the heat for a few minutes.

Season the rice

• Mix the rice vinegar, sugar and salt until dissolved.
• Fold the seasoning into the warm rice and let it cool slightly.

Prepare the filling

• Mix the drained tuna with the mayonnaise.
• Slice the cucumber into thin sticks and the avocado into slices.

Assemble the onigirazu

• Lay a nori sheet on a piece of plastic wrap and spread half of the rice in a square in the center.
• Top with the tuna mixture, cucumber and avocado, then cover with the remaining rice.
• Fold the nori corners over the filling, wrap tightly in the plastic wrap and press to shape.

Rest and slice

• Let the wrapped sandwich rest so the nori softens and holds together.
• Unwrap, sprinkle with sesame seeds and cut in half with a wet knife.