Remove the pits out of the umeboshi plums, and chop up with a knife. (I used bonito flavoured umeboshi. If the umeboshi is small, use about 6.)
Put Step 1 into a bowl, add the remaining ingredients, and mix.
Here's a recipe for chilled chicken slices with umeboshi, bonito flakes and soy sauce that uses this dressing (Recipe ID: 2284359).
Story Behind this Recipe
I tried making a sauce from umeboshi plum paste to perk up the flavors during the summer.
This recipe makes a large amount, so you can start with 1/2 the amount. Please adjust the amount of mirin to your liking depending on the kind of umeboshi you use. You can use this dressing as salad dressing, too!