Wash rice until the water runs clear. Add equal parts rice and water to a saucepan or rice cooker. If cooking on the stove, bring to a boil, then reduce to low heat. Cover and cook for about 15–20 minutes.
Once cooked and slightly cooled, season the rice with rice vinegar, sugar, and salt. Mix well and set aside.
For the tacos shells
Cook shrimp tempura according to package instructions.
In a bowl, whisk together egg and cold water until smooth. Slowly add flour, cornstarch, and salt, mixing until a smooth batter forms. Add a little more water if needed to thin it out.
Cut nori into circles. Add 1–2 spoonfuls of rice and, with wet hands, spread it evenly. On the other side of the nori, spread a thin layer of tempura batter, then press into panko to coat.
Heat oil over medium heat and fry the nori, panko side down, until golden brown. Transfer to a taco holder.
To assemble tacos
Fill each shell with avocado slices, shrimp tempura, cucumber, and green onions. Top with spicy mayo, eel sauce, sriracha, crispy onions, and sesame seeds if desired. Serve and enjoy.