While the rice cooks, combine the rice vinegar, sugar, and salt in a small bowl and microwave for about 20 seconds, stirring until the sugar and salt are fully dissolved.

Line an 8x8-inch pan with plastic wrap leaving enough overhang on the sides. Transfer the seasoned rice into the pan and press firmly into an even layer about 1.5 inches thick. Cover with plastic wrap, optionally weigh it down with a flat surface, and refrigerate for at least 4 hours or overnight for best results.

Cut the salmon into ½ inch cubes then finely mince with a knife until paste-like. Add the Japanese mayo, sriracha, soy sauce, sesame oil, chopped green onion, and salt. Mix until smooth and well combined. Cover and refrigerate until ready to use.

Once the rice is fully chilled, remove it from the pan and transfer to a cutting board. Use a sharp knife to cut into 18 rectangular blocks (6 columns by 3 rows).

In a large skillet, add about 1 inch of neutral oil to the pan then heat on medium-high until it reaches 350F (you can test the oil with wooden chopsticks and small bubbles should form around them.) Fry the rice blocks in batches for 3-4 minutes per side until golden brown and crispy. Transfer to a wire rack to drain.

To serve, top each crispy rice block with about 2 tablespoons of spicy salmon, then finish with black sesame seeds, jalapeno slices, and sliced avocado if desired.