Prepare the Sushi Rice:

1. Season the Rice: In a small bowl, combine rice vinegar, sugar, and salt. Mix until the sugar and salt are dissolved.

2. Combine with Rice: Gently fold the seasoned mixture into the cooked sushi rice, ensuring it is evenly distributed. Be careful not to mash the rice.

3. Cool the Rice: Allow the rice to cool slightly for easier handling.

Form the Rice Cakes:

1. Press the Rice: Line a baking sheet or shallow dish with plastic wrap. Spread the seasoned rice evenly over the plastic wrap, pressing down firmly to create a compact layer about ¾ inch thick.

2. Chill the Rice: Cover the rice with another layer of plastic wrap and chill in the refrigerator for at least 30 minutes, or up to 1 hour, to firm up.

3. Cut into Squares: Remove the rice from the refrigerator and carefully invert it onto a cutting board. Use a sharp knife to cut the rice into small squares or rectangles (about 1.5-2 inches).

Prepare the Spicy Tuna:

1. Combine Tuna and Seasonings: In a medium bowl, gently combine the diced sushi-grade tuna with mayonnaise, sriracha, sesame oil, and soy sauce. Mix well, being careful not to overmix, which can make the tuna mushy.

2. Adjust Spice Level: Taste the mixture and adjust the amount of sriracha to your desired level of spiciness.

Fry the Rice Cakes:

1. Heat the Oil: Pour a generous amount of neutral cooking oil into a large skillet or deep fryer, about ½ inch deep. Heat the oil over medium-high heat until it reaches approximately 350°F (175°C).

2. Fry the Rice Cakes: Carefully place the rice cakes into the hot oil in a single layer, being sure not to overcrowd the pan.

3. Fry until Golden: Fry for about 2-3 minutes per side, or until the rice cakes are golden brown and crispy. Use a slotted spoon or spatula to carefully flip the rice cakes and fry the other side.

4. Drain Excess Oil: Remove the fried rice cakes from the oil and place them on a wire rack or paper towel-lined plate to drain excess oil.

Assemble and Serve:

1. Top with Spicy Tuna: Place the crispy rice cakes on a serving platter. Spoon a generous amount of the spicy tuna mixture onto each rice cake.

2. Garnish: Sprinkle with toasted sesame seeds and thinly sliced scallions.

3. Optional Garnishes: Add avocado slices or finely chopped nori seaweed for extra flavor and visual appeal.

4. Serve Immediately: Serve the Nobu Crispy Rice Spicy Tuna immediately while the rice is still warm and crispy.