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Put the rice in a small bowl. Cover with cold water and massage with your hands to remove the starch. Drain and put the rice in a small saucepan with a tight-fitting lid. Cover with a fingertip depth of cold water and simmer over a medium heat with the lid on for 10 mins. Remove from the heat and leave to steam with the lid on for another 15 mins, then stir through the rice wine vinegar.
Meanwhile, mix the mayonnaise with the shichimi togarashi in a small bowl and set aside. Put the tuna on a chopping board, cut into roughly 1cm cubes, then sprinkle liberally with flaky sea salt.
In a large bowl, mix the sesame oil, soy sauce, lime juice and chilli flakes. Add the tuna chunks and stir well, so that every piece is coated in the dressing.
To assemble, put a mound of rice in two bowls. Top each with half an avocado, the tuna and dressing, cherry tomatoes and nori. Sprinkle over the chopped nuts and spring onions, and finish with a spoonful of the spicy mayo.