In a small bowl, stir together rice vinegar, sugar, and salt until dissolved. Gently fold this mixture into the warm cooked sushi rice.

Line an 8x8-inch baking dish with plastic wrap. Press the seasoned rice firmly into an even layer about 3/4-inch thick. Cover and chill in the refrigerator for at least 2 hours until firm.

In a chilled bowl, combine the finely diced raw tuna, Japanese mayonnaise, sriracha, sesame oil, and soy sauce. Mix gently until fully blended, then cover and refrigerate until ready to assemble.

Remove the chilled rice block using the plastic wrap overhang and cut into 12 equal rectangular pieces using a wet, sharp knife.

Heat neutral cooking oil in a large non-stick skillet over medium-high heat. Fry the rice cakes for 3 to 4 minutes per side until deeply golden and crispy. Transfer to a paper-towel-lined plate to drain.

Spoon a generous portion of the spicy tuna mixture onto each warm crispy rice cake. Top each piece with a slice of jalapeno, sliced green onion, and a sprinkle of toasted sesame seeds. Serve immediately.