Tsukimi Soba

The dish takes its name from tsukimi, Japan's autumn moon-viewing custom centered on the harvest full moon, and from a broader culinary convention in which a round, intact egg yolk stands for the moon. By that logic a raw egg dropped onto noodles turns kake soba into tsukimi soba, the same way it gives tsukimi udon, tsukimi ramen, and the tsukimi burger their names. The naming is a visual pun on the moon rather than a dish with a single recorded inventor, and it applies across many bowls topped with a whole egg.

Before you stir it, the bowl is meant to be looked at: the bright yolk sitting whole in the broth, the white half-set and translucent, nori dark at the rim. Then you break the yolk and fold it through the hot soup, where it thickens the broth into something silkier and rounder, coating the noodles and softening the dashi's edge. The first mouthful before stirring is clean broth and a slip of raw yolk; after stirring it is richer and gentler throughout.

The egg works because of how its proteins set with heat: egg-white proteins begin to coagulate at lower temperatures than the yolk, so the broth's heat firms the white into a soft cloud while the yolk, which sets higher, stays liquid. Stirring the raw yolk into the hot broth then emulsifies it — yolk lecithin disperses its fat through the liquid — which thickens and enriches the soup without curdling it, much as yolk thickens a sauce. Add the egg at the last moment over piping-hot broth; too little heat leaves the white slimy, while boiling it hard turns the yolk to a hard lump and loses the 'moon.'