Rinse sushi rice until water runs clear, then cook with water in rice cooker or pot.

Combine rice vinegar, sugar, and salt in a small saucepan over low heat until dissolved, then fold into cooked rice.

Mix diced tuna, mayonnaise, sriracha, sesame oil, and green onion in a bowl.

Lay bamboo mat covered in plastic wrap, place nori sheet shiny side down.

Spread rice evenly over nori, leaving 1 inch at top edge uncovered.

Add spicy tuna, cucumber, and avocado along the center.

Roll nori using bamboo mat, pressing gently, and seal edge with water.

Slice rolls into 6–8 pieces using a sharp wet knife.

Sprinkle sesame seeds if desired and serve with soy sauce, ginger, and wasabi.