Behead and gut the sardines, and wash away the blood. Wipe off the moisture. Mince the ginger into matchsticks, and use your hands to crush the umeboshi. Combine the ingredients, except for the sardines, in a pot and bring to a boil.
Once it comes to a boil, add the sardines, cover with the lid, and pressure-cook for 15 minutes. Then, let it sit until the gauge drops.
Once the gauge has dropped, open the lid, cover with a drop-lid, and simmer over medium heat until the sauce reduces by half.
Even the bones will be soft. The sweet and salty flavor goes great with rice.
Story Behind this Recipe
I came up with this way to get plenty of calcium.
I use sweet soy sauce from western Japan. If using a saltier soy sauce, it might be best to slightly reduce the amount. Also, I use medium-sized umeboshi. Be careful not to burn it while simmering.