Prepare the sushi rice.
In a medium bowl, whisk together mayonnaise and sriracha. Once combined, add in cubed tuna. Cover, and allow to marinate in the refrigerator while preparing the other ingredients.
Cut both sheets of nori in half. Lay one piece of nori shiny side down, on a bamboo mat covered in plastic wrap. Spread about ¾ cup of rice evenly on the nori sheet. If needed, wet hands to help spread the rice. If desired, sprinkle with ½ tablespoon of white or black sesame seeds, making sure they adhere to the rice.
Flip the nori over (rice side facing down). Place about 1 - 2 tablespoons of the tuna mixture along the bottom of the nori. Add a few slices of avocado right above the tuna.
Grab the nori and the rolling mat. Fold the nori over until it touches itself. Continue rolling until a complete roll is formed. Squeeze gently, but firmly. Repeat with remaining 3 nori sheets.
Transfer to a cutting board. With a very sharp knife, slice roll into 8 slices. Wipe knife on a damp cloth in between each cut to keep the rice from sticking.
Serve immediately with wasabi and pickled ginger.