This is easy to make and can be added to onigiri rice balls or fried rice, etc. It's convenient to make a lot and keep it for when you need it.
My mother taught me this recipe.
If you are not using umeboshi with a reduced salt content, soak them in water to remove excess salt before chopping. Of course, you don't have to do this if you prefer them on the salty side.
Enjoy Japanese food around the world.