Rinse the rice in a strainer until the water runs clear. Then, place into a pot with the water. Bring to a boil, stirring frequently.
Once boiling, cover and reduce the heat to low and simmer for 6 minutes. Check the rice. If there is no more water and just big grains of rice, it's ready. If not, cook another 2-3 minutes.
While the rice cooks, combine the vinegar, sugar and salt in a small container and microwave 30 seconds until the sugar is dissolved.
Transfer the rice into a PLASTIC container, using a WOODEN spoon (VERY important) and then pour the vinegar mix over top. Stir until well combined. Let the rice cool at room temp (not in the fridge) for 1-2 hours.
Mix all the ingredients for the tuna, except the tuna, in a medium bowl. Add the tuna and stir until well combined. Cover and refrigerate at least 15 mins but up to one hour.
Divide the cooled rice between 4 bowls, followed by the tuna. Finally, divide the cucumber and avocado and garnish with sesame seeds and nori, if using.
Mix the mayo and sriracha together until smooth and drizzle over the bowls.