Cook the Sushi Rice: Rinse the sushi rice in a fine-mesh sieve under cold running water until the water runs clear. This removes excess starch. Combine the rinsed rice and 400ml water in a saucepan. Bring to a boil over high heat, then reduce to a low simmer, cover, and cook for 15 minutes. Remove from the heat and let it steam, still covered, for another 10 minutes. You'll see the rice is tender and has absorbed all the water, smelling softly of cooked grains.

Season the Rice: While the rice is hot, transfer it to a wide, shallow bowl. In a small bowl, whisk together the rice vinegar, caster sugar, and salt until dissolved. Gently fold this mixture into the hot rice using a slicing motion with a spatula. Be careful not to mash the grains. Let the rice cool until it's just warm to the touch. The vinegar will give it a subtle, tangy fragrance.

Press and Chill: Line a small baking dish or a 20cm square tin with cling film, leaving some overhang. Drizzle 2 tablespoons of neutral oil over the base and sides. Press the seasoned rice firmly into the tin, creating an even, compact layer about 2cm thick. Fold the overhanging cling film over the top and press down firmly with another flat dish or a heavy can. Refrigerate for at least 1 hour, or until the rice is completely cold and firm to the touch. This step is crucial for the texture.

Prepare the Spicy Tuna: While the rice chills, make the topping. Finely dice the tuna into small, uniform cubes, about 0.5cm. In a mixing bowl, combine the diced tuna, mayonnaise, sriracha, toasted sesame oil, soy sauce, and most of the sliced spring onion (reserve a little for garnish). Gently fold everything together until the tuna is well coated. The mixture should look glossy and smell beautifully of sesame and heat. Cover and refrigerate until you're ready to assemble.

Fry the Rice Blocks: Once the rice is firm, lift it out of the tin using the cling film. Place it on a cutting board. Using a sharp knife, cut the block into 4 strips, then cut each strip into small rectangles or squares, roughly 4cm x 3cm. You should have about 20-24 pieces. Pour enough neutral oil into a large non-stick frying pan to reach a depth of about 1cm. Heat over medium-high heat. To test if it's ready, drop a small piece of rice into the oil; it should sizzle immediately. The oil should be shimmering but not smoking.

Sear to Golden Perfection: Carefully place several rice blocks into the hot oil in a single layer, making sure not to overcrowd the pan. Fry for 2-3 minutes on each side, until they turn a deep, even golden brown. You'll hear a satisfying, steady sizzle, and the edges will become visibly crisp and firm. The colour should be a rich amber. Use a slotted spatula to transfer them to a plate lined with kitchen paper to drain any excess oil. The sound of the sizzle will change as they cook, becoming slightly sharper as the crust hardens.

Assemble and Serve: Let the crispy rice blocks cool for just a minute—they should still be warm. Top each one with a small spoonful of the spicy tuna mixture. If using, add a tiny dice of avocado. Garnish with the remaining spring onion, a sprinkle of toasted sesame seeds, and furikake seasoning. Serve immediately. The contrast between the warm, crunchy base and the cool, creamy topping is the moment it all comes together.