Even though these marinated eggs are very well-flavored, the yolks remain soft since they aren't simmered. For fans of soft boiled eggs these are the ultimate in deliciousness. The reason they keep for so long is the vinegar!
Put the eggs in boiling water and cook for 7 minutes. Drain and put the eggs in cold water to cool them quickly.
Since the yolks are soft set, the key to peeling the eggs cleanly is to roll them around gently to crack the shells all over. Peel the eggs immersed in a bowl of cold water.
Mix the vinegar and mentsuyu sauce using a 1:2 ratio. Taste, and add a little water if it's too salty.
Put the cooked and peeled eggs in a bowl, an add enough marinade liquid to cover.
Marinate the eggs overnight. The eggs can be eaten the next day. Since they are marinated in a vinegar mixture, they will keep for about a week. The flavor will become stronger, and the yolk will set more.
Take the eggs out when they've reached your desired flavor and yolk consistency.
The eggs are great in a bowl of ramen noodles, with soboro (meat and other sprinkles) and rice, and in bentos. Top with some salmon caviar, flying fish roe, cream cheese and so on, to transform them into elegant hors d'oeuvres.
Story Behind this Recipe
I wanted to eat soft yolk eggs with a strong flavor. Simmered eggs become hard cooked. When I make simmered dishes like buta no kakuni (simmered pork belly) I often use the cooled leftover cooking liquid instead of the marinade here. Cool the cooking liquid down and remove the congealed fat before using as a marinade. The eggs become very rich in flavor, but really delicious. (In this case the eggs won't keep very long since there's no vinegar in the marinade.)
7 minute boiled eggs have pretty runny yolks. Please cook the eggs to your desired degree of doneness. The yolk will harden a little while it's marinating. The key to making the eggs last for some time is the vinegar in the marinade. If you don't like the sourness of vinegar, you can soften it a bit by bringing it to a boil once.