Rinse the rice: Place sushi rice in a fine mesh strainer and rinse under cool water until the water runs mostly clear.
Cook the rice: Cook in a rice cooker or simmer on the stovetop with the measured water, covered, until tender. Let it rest for 10 minutes.
Season the rice: Warm the rice vinegar, sugar, and salt just until dissolved (microwave or small pan). Fold into the warm rice gently. Let rice cool until just warm (not hot).
Make the spicy tuna: In a bowl, mix diced sushi-grade tuna, Japanese mayonnaise, sriracha, and soy sauce. Taste and adjust spice.
Prep your station: Wrap your sushi mat in plastic wrap. Set out a small bowl of water for your hands. Slice the cucumber and avocado.
Assemble the roll: Place 1 nori sheet shiny-side down on the mat. Wet hands and spread a thin, even layer of rice across the nori, leaving about 1 inch clear at the top edge.
Add filling: Lay the cucumber and avocado in a line across the lower third. Spoon spicy tuna in a neat line (don’t overfill).
Roll it up: Lift the mat edge and roll tightly away from you, pressing gently to shape. Seal the edge with a dab of water.
Optional topping: Sprinkle sesame seeds on the outside and gently press to adhere.
Slice: Use a very sharp, slightly wet knife. Cut the roll in half, then into 6–8 pieces. Wipe the blade between cuts for clean slices.
Serve: Plate with soy sauce, pickled ginger, and wasabi if you like.