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

Transfer the hot, cooked rice to a large non-metal bowl. Drizzle the seasoned rice vinegar evenly over the top and gently fold it in with a wooden spatula. Let cool to room temperature.

Line an 8x8 inch baking dish with plastic wrap, leaving overhang. Transfer the cooled rice to the dish and press it down extremely firmly into an even, tight layer.

Fold the plastic wrap over the top to cover the rice. Place in the refrigerator to chill and firm up for at least 4 hours, preferably overnight.

While the rice is chilling, prepare the tuna. Finely mince the sushi-grade tuna with a sharp knife.

In a bowl, whisk together the Kewpie mayonnaise, sriracha, and toasted sesame oil. Gently fold in the minced tuna until coated. Cover and refrigerate.

Remove the firm rice block from the fridge. Using a wet knife, cut the block into small, uniform rectangles (about 1x2 inches).

Heat the neutral oil in a heavy skillet over medium-high heat. Once hot, carefully add the rice rectangles. Fry for 3-4 minutes per side until deeply golden brown and crispy.

Transfer the fried crispy rice to a paper towel-lined plate to drain briefly.

To assemble: place a slice of avocado on top of each warm crispy rice block. Top with a generous spoonful of the chilled spicy tuna.

Garnish each piece with a single jalapeño slice and a sprinkle of sesame seeds. Serve immediately.