Combine all the ingredients, and leave for at least half a day or overnight.
Heat up over a low flame until just before it starts boiling. Heat gently for 10 minutes, taking care not to let it boil.
Strain the liquid though a tightly wrung out coarsely undyed cloth, and store in a clean jar in the refrigerator.
Story Behind this Recipe
I adapted a recipe that was in a weekly flyer. It's made to keep a long time since it has no water added, and dried shiitake mushrooms are added for a deeper and more complex flavor. There are lots of tasty commercial mentsuyu brands, but homemade is the best.