Corn, Fennel and Miso Soup

2 10-oz bags frozen sweet corn, or use fresh corn kernels, scraped from the cob
2 umeboshi plums, de-seeded if necessary and mashed into a paste
1 fennel bulb, trimmed and chopped finely
1/3 cup miso, or to taste
8 cups water
1 small onion, finely chopped

Combine everything in a large saucepan. Bring to a boil over high heat, then lower heat to a simmer and cook until fennel is tender, about 15 minutes. Serve hot.