Rinse the rice in a sieve under cold running water until it is clear. Drain well. Add the water and rice to a pan and leave to absorb for approx. 30 mins.
Bring the rice to the boil with the lid off, reduce the heat until the water has evaporated, leaving small indentations on the surface of the rice, cover and leave to absorb over a low heat for approx. 10 mins, do not lift the lid.
Warm the sugar, rice vinegar and salt in a small pan, stirring until the sugar has dissolved.
Pour the rice into a wide-bottomed bowl, drizzle with the warm liquid, mix carefully with a fork for approx. 3 mins. while fanning. Cover the rice with a damp cloth and leave to stand for approx. 15 mins.
Finely chop the salmon and coriander, place in a bowl and mix with the crème fraîche and lemon juice.
Place one nori sheet (rough side up) on a bamboo mat or piece of cling film. Place 1/3 of the rice onto the sheet, leaving a gap of approx. 2 cm at the top.
Cut the cucumber into sticks and the mango into thin strips. Place 1/3 of the salmon tartare, cucumber and mango in the centre of the rice. Repeat this process two more times.
Roll up the filled nori sheets, roll them between your hands until everything holds together. Slice the rolls into 8 equal pieces using a very sharp, wet knife.