Begin by cooking the rice. Wash it well in warm water, then place in a pan. Cover with water so that it comes 2cm/¾inch above the rice. Cover and cook gently for 12 minutes, or until the rice is tender and the water has been absorbed.
Meanwhile, mix together the vinegar, sugar and salt and set aside to dissolve. As soon as the rice is cooked, stir the vinegar mixture in to it and leave to cool completely.
Cut the cucumber, avocado and fish into batons.
Halve the nori sheets and place one half horizontally on the rolling mat. Spread 2-3 tbspof the mixture on the seaweed, leaving a 1cm/½in margin on the edge furthest from you.
Spread a tiny amount of the wasabi across the centre of the rice, then place a row of cucumber and avocado or fish across the centre. Roll up so the join is on the bottom of the roll. Continue to make rolls, alternating the ingredients so that some contain just fish and others contain cucumber and avocado.
Trim the ends of the rolls with a sharp knife, then cut each into six pieces. Transfer to serving dishes and place a pile of pickled ginger with each.