In a bowl, combine finely diced tuna with mayonnaise, sriracha, soy sauce, sesame oil, lime juice, and sliced green onions. Mix well.
**Make the Spicy Mayo:**
In a small bowl, mix together mayo, sambal, sriracha, sesame oil, sugar, and lemon juice until smooth and creamy. Set aside.
**Make the Seaweed Shells:**
Prepare the tempura batter by whisking together flour, cornstarch, baking powder, salt, black pepper, paprika, chili powder, and garlic powder in a bowl.
Add club soda and egg, then whisk until fully combined.
Heat vegetable oil in a deep frying pan to 350°F.
Dip one side of each nori square into the tempura batter, making sure to coat the edges as well.
Carefully place the nori in the hot oil, batter side down, and fry until crispy and golden.
Immediately fold the fried nori diagonally in half to create a taco shell shape, then let it drain on a wire rack. Repeat with remaining nori squares.
**Assemble the Tacos:**
Fill each seaweed shell with a layer of jasmine rice.
Add spicy tuna, avocado, and cucumber slices.
Drizzle with eel sauce and spicy mayo.
Sprinkle with toasted sesame seeds and green onions.
