Tuna mayo is the filling most people already associate with a rice ball, and the onigirazu version simply gives it more surface area. Inside the flat rice-and-nori frame, a pocket of tuna bound with Japanese mayonnaise sits as a single broad layer rather than a hidden core, so every bite carries the same ratio of fish, fat, rice, and seaweed. This is the variant that makes the strongest case for the format existing at all: the classic tsuna mayo onigiri is loved for its center, and flattening it spreads that center across the whole cut face.
The build follows the family pattern. A sheet of nori goes down shiny side toward the table, a thin even bed of lightly salted warm rice is pressed onto the middle, the tuna mayo is spread in a square that stops short of the edges, a second rice layer caps it, and the four flaps of seaweed fold in over the top so the parcel can rest seam side down before it is halved. With a wet filling the discipline is all about moisture control. Canned tuna is drained hard, sometimes pressed, before it is folded with mayonnaise so the mix is cohesive rather than runny, because anything loose will wick into the nori and turn the wrapper soft and prone to tearing. The filling is kept inside a clear margin so the rice can seal against rice at the perimeter and trap the tuna in the middle. A short wrapped rest lets the nori soften evenly into something pliable instead of brittle. Cut clean with a damp blade, the face shows a pale band of tuna framed by white rice and a thin dark seaweed line, and it eats cool, savory, and faintly oceanic, with the mayo doing most of the seasoning and the rice keeping it from feeling rich. Cut from a sloppy build, the tuna squeezes out the sides, the rice will not hold a shape, and the seaweed is already slick.
What the filling does inside the frame is make this the most forgiving member of the group. Tuna mayo holds its character cold for hours and does not depend on a crisp element the way a fried cutlet does, which is why this is the version that travels best in a lunch box and the one home cooks reach for first. Common touches stay within that logic: a layer of takuan or cucumber for crunch, sweetcorn folded into the mix, a smear of wasabi or a few shiso leaves for lift, all of them sharpening the same cool, mayo-led base.
The branches that earn their own names tend to be larger departures, swapping the wet salad center for grilled beef, a breaded cutlet, or pickled plum. Each of those rebalances the whole sandwich enough that it deserves its own article rather than being crowded in here.