I modified a recipe for mentsuyu that my mother taught me.
Step 1: Adjust the sugar to suit your tastes. If you don't have time, you don't need to let the ingredients sit in the fridge for a day. If you dry-fry/parch the simmered bonito flakes, they will become "furikake" (rice seasoning). Recipe ID: 1900964. You can also use this as soumen sauce. (Thin it out a little bit more for soumen than you would for soba.)