Rinse the sushi rice under cold water until the water runs clear. In a medium saucepan, combine the rinsed rice and water. Bring to a boil over medium-high heat. Once boiling, reduce heat to low, cover, and simmer for 20 minutes. Remove from heat and let it sit, covered, for an additional 10 minutes.

In a small bowl, mix the rice vinegar, sugar, and salt until dissolved. Once the rice has rested, transfer it to a large bowl and gently fold in the vinegar mixture. Allow the rice to cool to room temperature.

In a mixing bowl, combine the drained tuna, mayonnaise, and sriracha. Mix until well combined. Adjust the amount of sriracha to your desired spice level.

In a mold or ring, layer the ingredients in the following order: Start with a layer of sushi rice, pressing down gently. Add a layer of the spicy tuna mixture on top of the rice. Next, add a layer of diced avocado, followed by a layer of diced cucumber. Repeat the layers until you reach the top of the mold.

Carefully remove the mold or ring to reveal the sushi stack. Sprinkle with sliced green onions and sesame seeds. Serve with strips of nori on the side, soy sauce, pickled ginger, and wasabi for dipping.