We put the water, half a teaspoon of sugar and the same amount of salt in a saucepan. When the water starts to boil, add the rice.
Cook for 15 to 20 minutes in the boiling water, testing the rice until it is just right.
When the rice is ready, drain it, remove the water, wash it in cold water to remove the starch and leave it to cool.
While the rice is cooking, cut several strips of fashionable watermelon for the makisushi filling, about 1 centimetre wide and 5-6 centimetres long, and set aside.
Next, spread the mat on a flat surface and place a sheet of nori seaweed on top of it. On top of this and with wet hands, place a layer of rice that covers half of the nori seaweed.
On top and in the centre, place a sheet of tuna that takes up half of the rice and in the centre of the tuna, place one or more strips of the previously cut watermelon and the avocado strips. Now comes one of the most difficult steps: using the mat, roll up the seaweed, pressing with your fingers so that it is well pressed. Cut the makisushi into 2-3 cm thick portions and serve, accompanied by the soy sauce and wasabi and ginger to taste. Translated with www.DeepL.com/Translator (free version)