Heat water in a pot and add a little vinegar. Add an egg when it comes to a boil. When the egg is softly set, take it out and cool.
Cook the somen noodles.
Pour the mentsuyu into a bowl (it should be concentrated as the poached egg and somen noodles would dilute it)
Add the noodles and drop the egg in the middle. Chill in the fridge and serve cold!
Serve it chilled during the hot summer season. Add chopped scallions or ichimi spice if you prefer. If you don't have mentsuyu, combine cold dashi stock with soy sauce, mirin, bonito soup stock, and sake.