Rinse the sushi rice under cold water until the water runs clear, removing excess starch. In a rice cooker, combine the rinsed rice with water and a pinch of salt, then cook according to the rice cooker instructions until tender and sticky, about 20 minutes. Let it sit covered for 10 minutes to steam.

In a small bowl, mix rice vinegar, granulated sugar, and sesame oil until dissolved. Fold this mixture into the cooked rice using a spatula, ensuring the rice remains fluffy.

Transfer the flavored rice to a lined baking tray, pressing down firmly to create a block, about 1 inch thick. Cover with plastic wrap and refrigerate for at least 1 hour until firm.

Cut the rice block into rectangular pieces, about 2 inches wide. Heat oil in a skillet over medium-high heat until shimmering, then fry the rice rectangles for about 2-3 minutes on each side, until golden brown.

In a mixing bowl, combine the chopped Ahi tuna, Kewpie mayonnaise, Sriracha, soy sauce, sesame oil, and lime juice. Mix until well incorporated, adjusting spiciness as necessary.

Assemble by placing a slice of avocado on each rice rectangle, spooning a generous portion of the spicy tuna mixture over it, and sprinkling with sesame seeds, chives, and optional jalapeño.