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Put the rice in a bowl, cover with cold water and massage to remove the starch. Drain and repeat until the water runs clear. Put the rice in a small saucepan, cover with 2.5cm of water and put on a tight-fitting lid. Simmer on a medium heat for 10 mins, then take off the heat (leaving the lid on) and steam for a further 15 mins. Stir in the vinegar, then cool completely.
Toss the cucumber and carrot matchsticks in the soy and leave to marinate.
Lay out a sushi mat and put a nori sheet, shiny-side down, on top of it. Spread a quarter of the rice over the nori, leaving a 1cm border at the top. Mix the wasabi, ginger and lime juice.
Layer with the avocado, cucumber and carrot, and tuna. Top with the wasabi mix and coriander. Dampen the top border with a little water, fold in both sides of the nori sheet, then use the sushi mat to help roll. Wrap in foil, slice in half and serve.