Rinse the rice in water three times until clear. Combine with 400ml of water in a pan, bring to boil, then simmer for 15 mins until absorbed. Let it sit covered for 10 mins.
Fluff the rice with a fork, mix in rice vinegar. Spread in a lined baking tray to about 1cm thick, cover, and refrigerate for 3 hours or overnight.
Cut the chilled rice into 24 squares. Heat oil in a pan; cook squares for 4-5 mins on each side until golden. Drain on wire rack.
Assemble by spreading sriracha mayo on each square, topping with avocado, ponzu seasoning, ginger, nori, more mayo, and sesame seeds.
Notes
The rice squares can be prepared ahead of time; just fry them right before serving for optimal crispiness.