1.Put a sushi rolling mat (makisu) on your work surface, then put half a sheet of toasted nori seaweed on top. Take a handful of the rice (2-3 heaped tablespoons) in your hands and make into a log shape. Put the rice in the centre of the nori.
2. Using your fingers, spread it evenly all over, leaving about 1.5cm margin on the far side. (The rice will stick to your hands, so dip them in the hand vinegar first.)
3. Take a small dot of wasabi paste on the end of your fingeer and draw a line down the middle across the rice, leaving a light green shadow on top of the rice (not too much – wasabi is very hot).
4. Arrange 1 strip of cucumber across the rice, on top of the wasabi.
5. Pick up the mat from the near side and keep the cucumber in the centre.
6. Roll the mat over to meet the other side so that the rice stays inside the nori.
7. Lift the top edge of the mat. Press and roll the cylinder slightly. The join should be underneath so it will stick well (it sticks together because of the moisture in the rice.) Remove the cylinder from the mat and put, join side down, in a covered flat container while you make the remaining rolls.
8. Cut each roll in half, then each half into 3, making 6 pieces. Arrange on a serving plate and serve with pickled ginger, a little pile of wasabi paste and a dish of Japanese soy sauce.