Put all the sauce ingredients as well as the kombu and bonito flakes in a pan, simmer for about 2 minutes to cook off some of the alcohol and slightly concentrate the sauce. Leave for 30 minutes.
Strain the sauce through a fine-mesh tea strainer or paper towel. This is enough mentsuyu for 4 servings.
Cook the udon noodles. If using frozen noodles as I did here, just heat them up in the microwave.
If you're serving the noodles cold, rinse in cold water to cool. If you're serving them warm, keep warm after microwaving.
Put the noodles on a serving plate, add a tablespoon of sauce per serving, and garnish with the toppings. Drop an egg yolk on top to finish.
Mix everything together well before eating. Enjoy this cold in the summer and hot in the winter, or however you like.
This is the orginal udon bowl that inspired this recipe.
My family gave rave reviews for my version. It's easy, so give it a try.
Postscript: The taste will vary a lot depending on the soy sauce you use. We use a rather sweet soy sauce. Taste and adjust the amount of sugar to your taste. The aim is to create a sweet-salty mentsuyu.
Story Behind this Recipe
This is an imitation of the famous bukkake udon from Furuichi (the name of a restaurant) in Kurashiki-city. I've been eating this Kurashiki speciality since I was a child. I was inspired by user "Pecchin-san's" tip to use tamari for her Ise Udon recipe, and was able to make a version of bukkake udon sauce I was happy with, so I uploaded it!
Measure the ingredients for the sauce by adding them to a measuring cup. It's easier that way. From 110 ml of sauce you can get enough sauce for 4 servings, or 70 ml (4 tablespoons, 1 per serving). Use microwaveable frozen udon noodles to save time, and follow the package instructions to heat.