In a small bowl, mix rice vinegar, sugar, and salt until dissolved. Pour over the warm cooked sushi rice and gently fold together. Let cool slightly.

Divide the seasoned rice into four equal portions. Using wet hands or a mold, firmly press the rice into tight, compact 3/4-inch thick patties.

Wrap the patties in plastic wrap and chill in the refrigerator for at least 30 minutes to firm up.

In a mixing bowl, combine the chopped tuna, Kewpie mayo, sriracha, soy sauce, and 1/2 tsp sesame oil. Mix gently and keep refrigerated.

Heat neutral oil and 1 tbsp sesame oil in a large skillet over medium-high heat. Fry the chilled rice patties for 3-5 minutes per side until deeply golden and crispy. Drain on paper towels.

Assemble by placing one crispy rice bun on a plate, topping generously with the spicy tuna mixture, sliced avocado, and chopped scallions. Top with a second rice bun and serve immediately.