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First, make the rice. Rinse the sushi rice in a colander, massaging it with your hands until the water runs clear. Leave to drain for 15 mins.
Put the rice in a saucepan with 200ml water and the sake or mirin, if using. Bring to the boil, then cover with a lid, reduce the heat to low and simmer for 20 mins until the liquid is absorbed. Remove from the heat. Set aside, covered, for 15-20 mins.
Transfer to a large bowl. Dissolve the sugar, if using, in the vinegar with a good pinch of salt, pour over the rice and mix. Cover with a damp tea towel and set aside at room temperature until ready to use.
Put 1 nori sheet on a bamboo mat and cover the surface with half of the cooled rice. Lay half of the salmon and cucumber slices along the length of the rice in a strip, being careful not to overfill it. Run a pea-sized blob of wasabi (more if you like it really hot) along the edge of the filling with your finger.
Roll up tightly and squeeze to seal when you reach the end. Repeat with the remaining nori, salmon and cucumber, and more wasabi. Slice each roll into 6 pieces, and serve with extra wasabi, ginger, soy sauce and salmon roe, if you like. Will keep in the fridge for 1 day.