In a bowl, mix mayo and sriracha. Set aside.
In another bowl, combine cooked rice and rice vinegar. Cover and set aside.
In a third bowl, drain tuna of all water and combine with half of the sriracha mayo. Leave the remainder for drizzling at the end.
To assemble, take a 1/2 cup measuring cup and sprinkle sesame seeds into the bottom if desired. Next, layer 1/3 of the cumber, carrot, and avocado. spread 1/3 of the tuna on top and press down evenly with a spoon or spatula. Lastly, add 1/3 cup of sushi rice.
Flip the cup onto a plate and tap to loosen, slowly pulling the cup straight up to release stack.
Drizzle with remaining sriracha mayo and sprinkle with more sesame seeds.