Season the white fish fillet with salt and pepper.
Coat the fish fillet in flour, then egg wash, then breadcrumbs, in order.
Heat cooking oil in a pan over medium-low heat and fry the fish until golden brown.
Mix peanut butter, mayonnaise, lemon juice, soy sauce, and sugar to make peanut sauce.
Cut the bread in half and toast it.
Spread peanut sauce on the toasted bread.
Place the fried fish cutlet, cilantro, iceberg lettuce, onion, and cucumber on the bread.
Top again with peanut sauce, cover with the other slice of bread, and finish the sandwich.