Rinse sushi rice under cold water until clear, then cook according to package instructions or in a rice cooker, about 20 minutes. Let it rest for 10 minutes covered.

Mix rice vinegar, granulated sugar, and kosher salt in a small bowl until dissolved. Set aside while rice cooks.

Drizzle sushi vinegar over warm rice and fold gently to coat each grain. Be careful not to mash the rice.

Allow the seasoned rice to cool to room temperature for 20 minutes, then pack into a lined container. Refrigerate for at least 3 hours.

Finely dice ahi tuna and mix with sliced scallions in a bowl.

Whisk together Kewpie mayonnaise, Sriracha, and ponzu sauce until smooth, then fold into the tuna and scallions. Chill mixture.

Cut firm rice into rectangles. Heat grapeseed oil in a skillet over medium heat. Fry rice for about 2 minutes per side until golden brown.

Drain fried rice on paper towels. Top with spicy tuna mixture, garnish with black sesame seeds and optional jalapeño slices before serving.