How to Make It
Combine 5 cups water and kombu in a medium saucepan; bring to a boil. Remove from heat; let stand 20 minutes. Discard kombu. Add bacon to broth; bring to a boil. Reduce heat, and simmer 30 minutes. Strain broth through a cheesecloth-lined sieve into a bowl. Skim any fat from the surface; discard fat.
Return broth to saucepan; bring to a boil. Add mushrooms; reduce heat, and simmer 3 minutes. Stir in bok choy, soy sauce, and vinegar. Garnish with cilantro, if desired. Serve immediately.