Rinse the sushi rice under cold water until it runs clear. Cook with the water according to package directions or in a rice cooker.

In a small bowl, whisk together the rice vinegar, sugar, and salt until dissolved. Fold this mixture into the warm cooked rice.

Line a square container with plastic wrap. Press the seasoned warm rice firmly into an even, tightly packed 3/4-inch thick layer. Cover and chill in the refrigerator for at least 2 hours until very firm.

Finely dice the raw sushi-grade tuna. In a bowl, mix the Kewpie mayo, Sriracha, sesame oil, and soy sauce. Fold in the diced tuna and chopped scallions until coated. Keep refrigerated.

Remove the chilled rice block from the fridge and cut it into bite-sized rectangles using a wet knife.

Heat neutral oil in a skillet over medium-high heat. Pan-fry the rice rectangles for 2-3 minutes per side until deep golden brown and crispy. Drain on paper towels.

Immediately top each piece of warm crispy rice with a spoonful of the chilled spicy tuna mixture. Garnish with a jalapeño slice, black sesame seeds, and extra scallions. Serve right away.