Photograph: Gareth Morgans
Soak the rice in cold water for 30 minutes, then drain and place in a medium saucepan with 850ml water. Cover and bring to the boil. Turn the heat down and simmer gently for 10 minutes. Stir in the sushi vinegar then spread the rice out on a baking sheet and leave to cool.
Peel the avocado, slice into long, thin strips and toss with the lemon juice. With wet hands, spread ¼ of the cooled rice on to a piece of nonstick baking paper the same size as a nori sheet. Lay a sheet of nori on top of the rice, then arrange ¼ of the avocado strips in a line along the long bottom edge, followed by ¼ of the crabmeat. Lift the edge of the paper and, using it to help, start to roll the sushi to create a cylinder, keeping the filling in place with your fingers as you go. (Don't roll the paper into your sushi!)
Sprinkle the roll with sesame seeds and use a damp knife to cut into 2cm-thick slices. Repeat with the remaining ingredients, using a new sheet of baking paper each time. Serve with pickled ginger, wasabi and soy sauce.
Get ahead: make a few hours ahead; chill.
Kitchen secret: lemon juice stops the avocado from oxidising, and adds an extra tang to the rolls, too.