Rinse and swirl rice with cold water in a large pot, with a tight fitting lid. Drain in a fine mesh sieve. Repeat this step a few times, until the water is no longer cloudy and white.
Return the rice to the pot, add the water, and bring to a boil over high heat.
Once the water begins to boil, immediately reduce the heat to the lowest setting, place the lid tightly on the pot, and cook for 20 minutes without touching the lid.
After 20 minutes, leave the lid intact, turn off the heat, remove from the burner, and let stand for 10 minutes to complete cooking and reach an ideal sticky texture.
While the rice is cooking, in a large mixing bowl, add the tuna, mayonnaise, salt, pepper, and sriracha. Stir well to combine, mashing the tuna chunks if needed in order to separate. Reserve to the side until needed.
Once the rice is cooked, add vinegar, the remaining 1/2 of the salt, and sugar. Gently stir to combine and fluff with a fork.
Distribute an equal amount of rice into each bowl, top with even amounts of the spicy tuna, carrots, cucumber, avocado, and scallions. Garnish each bowl with sesame seeds and extra sriracha if desired.