Place rice, water and rice vinegar in a pot over high heat and bring to the boil. Cover with a lid and simmer for 10 min, until rice is tender and set aside to cool down.
In a small bowl, mix tuna with mayonnaise, reserve.
Set up your bamboo rolling mat or alternatively a thick, lint-free kitchen towel and line with plastic wrap or baking paper.
Place a piece of nori seaweed in the bottom centre of your mat.
Add 1 tbsp of rice and using wet hands spread along the nori leaving a 1cm gap with no rice on the top.
Add 1 tbsp of the tuna across horizontally on centre of the nori sheet.
Add a couple of slices of avocado above the tuna, making sure there is enough filling edge to edge.
Gently lift the bottom of the mat up and over the sushi. Press and shape the ingredients into a tube. Roll with pressure so you get a firm roll.
Roll until just an inch of nori shows at the top and seal the edge of the nori with a little cold water.
Firm it up by squeezing the mat around the roll until it feels uniformly snug.
Slice the sushi to bite size pieces with a very sharp knife.
TIP: For grown up Sushi, you can use 1 half sheet of Nori and add raw salmon instead of tuna.
