Rinse the sushi rice under cold water until the water runs clear, then cook with the water in a rice cooker or stovetop pot until tender.

While the rice cooks, whisk together the rice vinegar, sugar, and salt in a small bowl until the sugar is completely dissolved.

Transfer the hot cooked rice to a large wooden or glass bowl and gently fold in the vinegar mixture, fanning the rice to cool it down to room temperature.

In a separate bowl, combine the diced sushi-grade tuna with mayonnaise, sriracha, and sesame oil, mixing until the fish is evenly coated.

Place a bamboo rolling mat on a flat surface, wrap it in plastic wrap for easier cleanup, and place a sheet of nori on top shiny-side down.

Wet your hands to prevent sticking, then spread a thin, even layer of sushi rice over the nori, leaving about an inch of empty space at the top edge.

Arrange a line of the spicy tuna mixture and a few avocado slices horizontally across the center of the rice layer.

Lift the bottom edge of the bamboo mat and roll the sushi tightly away from you, tucking the ingredients in as you roll to form a compact cylinder.

Slice the roll into 6 to 8 pieces using a very sharp, wet knife to ensure clean cuts without crushing the rice.

Sprinkle the rolls with toasted sesame seeds and serve immediately with soy sauce, pickled ginger, and wasabi.