In a large mixing bowl, combine your cooked sushi rice with rice vinegar, sugar, and salt. Use a gentle folding motion to ensure the rice absorbs the liquid evenly. Allow the seasoned rice to cool to room temperature for 10-15 minutes.

With damp hands, grab a handful of the cooled rice mixture and press it into rectangular blocks (1 inch by 2 inches). Place each block on a parchment-lined tray, leaving space between them.

Heat vegetable oil in a skillet over medium-high heat until shimmering. Add rice blocks in batches, frying for about 2-3 minutes until golden-brown. Transfer to paper towel-lined plate to drain excess oil.

Dice sushi-grade tuna into small pieces. In a mixing bowl, add mayonnaise, Sriracha, soy sauce, toasted sesame oil, and green onion. Mix gently until combined.

Top each crispy rice block with a spoonful of the spicy tuna mixture, and garnish with optional toppings like sesame seeds or jalapeños.

Serve immediately while the rice is still crispy, with dipping sauces if desired.