To cook the rice, start by washing it very well. Using a fine mesh strainer, rinse the rice under cool running water for 2 minutes, or until the water runs clear. Drain the rice.
Combine the drained rice with water in a saucepan, bring to a boil, cover with the lid and reduce the heat to low, and simmer for 15 minutes without removing the lid. Remove from heat, and do not touch it for 5 minutes. Then remove the lid and fluff it with a wooden spoon.
In a small bowl or jug, combine the sushi rice seasoning (rice vinegar, sugar, and salt). If needed, warm the mixture in the microwave or over the stove so that the sugar is fully dissolved.
Add the mixture to the rice, and stir with a wooden spoon. Set aside, and allow to cool a little.
To make the spicy mayo, in a small bowl combine the mayo with sriracha, and set aside.
To assemble the bowl, divide the rice between 4 bowls, and top with crab, cucumber, avocado, carrot, and nori sheets. Sprinkle with sesame seeds, and drizzle with spicy mayo.
Serve with soy sauce on the side.