Rinse sushi rice under cold water until clear, then cook as per instructions. Combine rice vinegar, sugar, and salt in a saucepan over low heat until dissolved. Mix with warm rice and set aside to cool.

Line a dish with plastic wrap, press rice into a ½-inch layer, cover, and refrigerate for at least 3 hours or overnight.

Finely chop ahi tuna, mix with scallions. In a separate bowl, mix mayo, sriracha, and ponzu. Combine tuna with spicy mayo mixture and refrigerate.

Heat grapeseed oil in a skillet over medium-high heat. Cut rice into rectangles, fry for about 2 minutes per side until golden brown and crispy. Drain on paper towels.

Top crispy rice with spicy tuna mixture, sprinkle with sesame seeds and jalapeño slices. Serve immediately.