Preparing the beans:
Sort, wash, and strain the kidney beans, place in glass bowl, add water and 1/2-inch piece of kombu, soak overnight (8 to 12 hours). The next day, discard soaking water, put kidney beans in large pot, add 6 cups of water. Bring to a boil over high heat, reduce heat to medium and boil for ten minutes, skim off foam. Once the foaming process has settled down, cover and reduce heat to low, simmer for 45 minutes.
Mix oil, vinegar, and red onions together in a bowl (if using umeboshi paste or plums, also add now). Add kidney beans to the dressing while warm, this allows the beans to absorb the flavor faster.
This recipe can be served immediately but if you give it time to marinate it is even tastier.

1 cup kidney beans
4 cups water
1 piece of kombu, (the size of a postage stamp) or 1 pinch of sea salt
1 cup red onion minced
2 tablespoon olive oil (or 1 tablespoon toasted sesame oil and 1 tablespoon olive oil)
3 tablespoon ume vinegar
optional: add 2 tablespoon umeboshi paste or 2 chopped umeboshi plums