Drain the tuna and add to a medium sized bowl. Flake apart with a fork.
Add kewpie mayo, sriracha, and diced green onions to the tuna and mix until well combined.
You can adjust the mayo and sriracha ratio to meet your preferences.
Starting with room temperature or slightly warm sushi rice, add a scoop of rice to the onigiri mold and then add a small amount of the spicy tuna mixture and avocado to the center.
Top it with more rice (until the filling is covered) and use the top of the onigiri mold to press it into shape.
Remove the onigiri from the mold and press a small piece of seaweed across the bottom.
Place black sesame seeds on a plate and turn the onigiri in it until the edges are covered.
Serve with soy sauce and enjoy!