Combine the rinsed sushi rice and water in a medium saucepan. Bring to a boil over medium-high heat, cover tightly, reduce heat to low, and cook undisturbed for 15 minutes. Remove from heat and allow to steam covered for 10 minutes.

In a small bowl, whisk together the rice vinegar, sugar, and 0.3 teaspoon salt until dissolved. Transfer warm rice into a wide non-metallic bowl, drizzle the seasoned vinegar evenly over it, and gently fold with a rice paddle to coat. Let cool to room temperature.

Whisk together the mayonnaise and sriracha in a small ramekin until smooth. Set aside for drizzling.

Prepare the tempura batter by whisking together the flour, cornstarch, remaining 0.3 teaspoon salt, and chilled ice water just until combined; small lumps are completely fine.

Heat the frying oil in a wide skillet or wok to 177 °C (175 °C). Dip the shiny, smooth side of each nori sheet lightly into the batter. Carefully lay into hot oil and immediately use tongs to bend the sheet into a taco shell shape as it crisps. Fry for 45 to 60 seconds until firm and bubbly, then drain on a wire rack upside down.

Assemble the tacos by gently spooning 2 to 45 ml of seasoned sushi rice into the base of each crispy nori taco shell.

Top the sushi rice layer with cucumber strips and generous slices of ripe avocado. Drizzle with spicy mayo, then garnish with sliced green onions and toasted sesame seeds. Serve immediately.