Two ingredients. Egg and dashi. The stock is brought to a boil, beaten egg is poured in, and the lid goes on immediately. After that nobody touches anything. The foam that rises is allowed to set as foam, which is the entire technique and also the entire difficulty.

It was served at breakfast to travelers staying at the Ota waki-honjin in Fukuroi, a post station on the Tokaido road, during the Edo period. The name appears in a travel diary called the Sendai Geko Nikki. Then it disappeared for a long time, the way inn food does, and in 2006 the local tourism association reconstructed it from old cookbooks.

What arrives is a lacquer bowl with a pale dome of foam standing above the rim. It does not look like a dish so much as a weather condition. I hesitated before putting a spoon in, which felt faintly ridiculous, given that it was breakfast.

There is no resistance at all. The foam goes and leaves the smell of dashi behind, and then you are looking at an empty bowl wondering whether you ate anything. Travelers two hundred years ago were leaning over the same bowl in the same town, at the halfway point of a road they were walking the whole length of. It would have been the lightest thing they ate all week.