Onigirazu (Sushi Sandwich)

Make the tuna salad: Place tuna in a medium bowl; use a fork to flake into large chunks. Add Kewpie mayonnaise, scallions, soy sauce, and sriracha; stir to combine. Set aside or refrigerate until ready to use.

Assemble the onigirazu: Toss together carrots, cucumber, and vinegar in a small bowl; set aside.

Place 1 nori sheet on a clean work surface, shiny side down; using kitchen scissors, starting on bottom edge of nori, cut a vertical slit, cutting sheet in half and stopping in center of nori.

In bottom right corner, spread 1/4 cup of the rice in an even layer. Place half of the carrot mixture, about 2 tablespoons, in top right corner, spreading into even layer. Fan half of the avocado (about 1/4 cup) diagonally in top left corner. Place half of the tuna salad, about 1/4 cup, in the bottom left corner, spreading into an even layer. Beginning at the rice corner, fold rice over the carrot mixture, then fold over the avocado corner, and finish by folding over the tuna salad corner. Wrap loosely in plastic wrap and set aside.

Repeat process with remaining 1 nori sheet, 1/4 cup rice, half carrot mixture, half avocado, and half tuna salad. Refrigerate at least 30 minutes or up to 24 hours before slicing in half to serve. Note: Keep plastic wrap on to help slice cleanly, but remove before eating.