Heat olive oil in a saucepan. When hot, add the onion, cover and cook over low heat for 5 minutes or until tender. Add the garlic and cumin and saute for 10 seconds to release their aroma. Add the broth and water and bring to a simmer.
While the liquid is coming to a simmer, with a knife separate the cilantro leaves from the stems. If you wish, wrap the stems well with string and add to the broth to infuse the liquid with more cilantro flavor. When the liquid is at a simmer, add the corn and beans and simmer for 5 to 10 minutes just to heat the ingredients well. Meanwhile chop the cilantro leaves you need anywhere from 1/2 to 1 cup of chopped leaves. Add the cilantro leaves and immediately remove the soup from the heat. Discard the cilantro stems. Add the lime juice and season to taste with salt and cayenne pepper. Ladle the soup out and garnish each portion with sour cream and garnish the sour cream with pimento. Variation: to turn this into a main course soup, add 12 ounces of strips of raw chicken when you add the beans and corn. Or add 12 ounces chopped cleaned shrimp after you have added the beans and corn and cook for 2 minutes to cook through.