No onigirazu divides a room like the natto one. Fermented soybeans, sticky, pungent, drawn into long glossy threads when stirred, are a breakfast staple across much of Japan and a hard wall for many people elsewhere. As a filling they make the rice-and-nori frame do something none of the other versions ask of it: hold a wet, stringy, strongly flavored ingredient together long enough to cut a clean face.
Construction follows the format and gains one quirk from the filling. A square of nori on the diagonal, a flat bed of seasoned rice, the natto spread across it, a second even rice layer, corners folded to a sealed packet, a rest seam-side down, a clean halving. The quirk is the stickiness. Natto is loose and adhesive, so it is almost always seasoned first with its usual companions, the sachet of karashi mustard and tare soy, sometimes chopped scallion, and then often bound with something that gives the band more structure: a layer of rice that has been pressed a touch firmer than usual, or a partner ingredient that holds a shape. The cut is where natto misbehaves, the threads trailing off the knife and across the face, so the blade is wetted and wiped between strokes more attentively than for any other version in the line.
The honest faults are particular to fermented soy. Too much natto and the packet is a sticky mass that smears rather than slices and overwhelms everything with one note; too little and the assertive flavor is the only thing in an otherwise plain rice packet, with nothing to play against. The reliable builds pair it with something that interrupts: raw egg or tamagoyaki for roundness, kimchi for heat and crunch, shiso or scallion for a green edge, sometimes cheese for an unexpected savory bridge. Drier natto, mixed less vigorously, behaves better in a packet meant to travel than a heavily whipped, maximally stringy version. None of this softens the central truth: this is an acquired taste, and the format does not domesticate it so much as give it a frame.
Variations are mostly about the partner that tempers it. Kimchi-natto is the most common, trading on heat against funk; natto with egg leans soft and mellow; natto with avocado doubles down on creamy texture; natto with cheese is the most divisive of an already divisive set. Each pairing is a different proposition and deserves its own article rather than being crowded in here.