Cook the Sushi Rice: First, rinse the sushi rice in a sieve under cold running water until the water runs clear. Add the rinsed rice and 400ml of cold water to a medium saucepan. Bring to a boil, then immediately reduce the heat to the lowest setting, cover with a tight-fitting lid, and simmer for 15 minutes. Do not lift the lid during this time. After 15 minutes, remove from the heat and let it stand, still covered, for another 10 minutes. For a more detailed guide on perfecting this step, Serious Eats has a great tutorial.

Prepare the Sushi Vinegar: While the rice is resting, gently heat the rice vinegar, caster sugar, and salt in a small pan over a low heat. Stir until the sugar and salt have completely dissolved. Do not let it boil. Set aside.

Season the Rice: Transfer the cooked rice to a large, non-metallic bowl (a wooden or glass one is ideal). Pour the sushi vinegar mixture evenly over the hot rice. Using a spatula or rice paddle, gently 'cut' and fold the vinegar through the rice. Try not to stir or mash the grains. Allow the rice to cool until it's just warm enough to handle – about 15-20 minutes.

Make the Tuna Filling: In a separate bowl, flake the well-drained tuna with a fork. Add the Japanese mayonnaise, soy sauce, finely sliced spring onions, and toasted sesame oil. Mix everything together until it's well combined and creamy.

Shape the Balls: Prepare a small bowl of salted water to dip your hands in – this is the secret to stopping the rice from sticking to you! Take a handful of the seasoned rice (about 60-70g) and flatten it into a circle in the palm of your hand. I find that creating a small indent in the middle helps with the next step.

Fill and Seal: Place a heaped teaspoon of the tuna filling into the indent in the rice. Bring the edges of the rice up and around the filling, enclosing it completely. Gently cup the rice in your hands and press firmly, shaping it into a tight ball or a traditional triangular onigiri shape.

Coat and Serve: Roll the finished Tuna Rice Ball in a shallow dish filled with toasted sesame seeds until it's lightly coated. Repeat the process with the remaining rice and filling. They can be enjoyed immediately or stored for later.