Warm vinegar, sugar, and salt in a pan until dissolved, then cool.
Wash rice three times, drain, and let stand for 15 minutes.
Boil 1 litre of water with kombu and rice, then simmer for 10 minutes.
Let rice stand off the heat with the lid on for 20 minutes.
Transfer rice to a bowl and slice in the sushi vinegar with a spatula.
Fan the rice for 10 minutes, turn it over, and fan for another 2 minutes.
Slice fresh sushi-grade fish into thin strips for the nigiri.
Butterfly the prawns and impale them lengthways on skewers.
Simmer prawns in 500ml of water and sake for 2 minutes until pink.
Shape walnut-sized balls of rice gently with your hands.
Apply a dot of wasabi and a fish strip or prawn to the rice balls.
Place half a nori sheet on a cling-film covered bamboo mat.
Spread rice evenly, leaving a 2cm border at the edge.
Add filling strips and roll the mat tightly away from you.
Slice rolls into bite-sized pieces with a damp knife and serve with ginger, wasabi, and soy.