DRAIN THE BEANS and put them in a soup pot with enough cold water to cover by a couple of inches. Bring to a boil and skim off the foam that rises to the surface; then add the onions and the garlic. Lower the heat and cook until the onions are soft, about 15 minutes; add the tomatoes and their juice, the pureed chiles and half the cilantro and lightly salt. Simmer until the beans are tender, an hour or so. Occasionally give them a stir while they're cooking. When done, taste for salt, stir in the remaining cilantro and garnish as desired.