The tonkatsu onigirazu takes the flat rice-and-nori frame and asks it to hold something it was never sized for: a breaded pork cutlet. Where the baseline onigirazu keeps its filling thin and tidy, this version commits to a slab of tonkatsu, fried until the crust is crisp and the pork inside stays juicy, then glossed with thick brown tonkatsu sauce. The whole thing is a flatter, rice-bound answer to the katsu sando, and the trade is deliberate: you give up the soft shokupan and gain a base of seasoned rice that absorbs sauce instead of being soaked by it.
The frame is the same one the whole family shares. A square of nori is laid out shiny side down, a thin even bed of warm rice is pressed onto its center, the cutlet and sauce go on top, more rice caps it, and the four corners of the seaweed are folded in like an envelope so the package is sealed seam side down and left to settle before it is cut. With tonkatsu the rice layers matter more than usual. They have to be thin enough that the sandwich still reads as crust-and-pork rather than a rice ball with a surprise inside, but firm enough to keep the cutlet from sliding when the knife comes down. The cutlet itself is usually a rosu loin or a leaner hire fillet, sliced to a thickness the nori can actually wrap, with the sauce kept restrained so the nori does not turn slick and tear. Done well, the cut face shows a clean stripe of golden crust and pink-white pork banded by white rice and a dark seaweed edge, and the first bite gives crunch, then fat, then the faint salt of toasted seaweed. Done sloppily, the sauce soaks through, the nori splits at a corner, and the crust goes soft before the package is even cut, leaving something closer to wet rice than a sandwich.
What the filling does inside the frame is supply contrast the plain versions do not have. The fried crust is the only crisp element in an otherwise soft, cool, pressed object, so the sandwich lives or dies on whether that crust survives contact with rice and sauce. This is why it is best assembled and eaten reasonably promptly rather than held for hours like a tuna or umeboshi build, and why some makers fry the cutlet a shade harder than they would for a plate so it has crunch left to give once it is wrapped.
Branches off this version mostly swap the protein while keeping the rice-and-nori envelope: a chicken katsu fill, an ebi fry fill, a menchi katsu fill, or a curry-sauced cutlet that pushes the whole thing toward a hand-held katsu curry. Each changes the balance of crust, fat, and sauce enough that it deserves its own article rather than being crowded in here.