Preheat the broiler: Place a rack closest to the broiler and heat it to high. In a medium bowl, combine mayonnaise, sriracha, sesame oil (if using), and 4 teaspoons soy sauce.

Prepare the salmon: Place the salmon fillets on a large foil-lined baking sheet and season all over with 1 teaspoon salt. Spoon 1 tablespoon of the spicy mayo on each fillet and rub it all over. Broil the salmon, watching closely, until the tops start to brown and blister (9 to 11 minutes).

Prepare the rice and cabbage mixtures:

• In a large bowl, toss the sushi rice, furikake, 1 tablespoon rice vinegar, 1 teaspoon sugar, and the remaining 2 teaspoons soy sauce until combined.

• In a small bowl, toss the shredded cabbage, 1/4 teaspoon salt, the remaining 2 tablespoons rice vinegar, and 1/4 teaspoon sugar until combined.

Break down the salmon: Once the salmon is cooked, transfer it to a medium bowl and add 2 tablespoons of spicy mayo. Using a fork, break the salmon into small pieces and toss until coated.

Assemble the sushi sandwiches:

• Place a sheet of nori with a point facing you on a work surface. Spoon 1/3 cup of the rice mixture into the center and use wet fingers to pat the rice into a square.

• Top with 1/4 cup of the salmon mixture, spreading it over the rice. Layer with 4 slices of cucumber, 2 tablespoons of cabbage mixture, 1 tablespoon of grated carrot, 2 slices of avocado, and 2 tablespoons of spicy mayo.

• Fold the top corner of the nori down over the center. Repeat with the bottom, left, and right corners to make a square pouch, enclosing the fillings. Tightly wrap with plastic wrap. Repeat with the remaining sheets and filling. Let sit for 10 minutes.

Serve: Unwrap the sushi sandwich and slice it in half. Serve with the remaining spicy mayo alongside for dipping.