Place the beans in a big heavy pot and cover with enough water to cover the beans by at least 3-inches, about 12 cups of water. Incorporate the onion and bring to a rolling boil. You may also add cilantro or epazote. Let the beans simmer over medium heat, partially covered, for about 1 hour and 15 minutes, until the beans are soft and then add the salt. Don’t add the salt in the beginning, or it will toughen the beans.
Let them continue simmering for another couple minutes, or until the beans are so soft, they come apart if you hold one between your fingers, and the broth has thickened to a soupy consistency. If the beans are not yet soft and the broth is drying out, add more hot water. Remove the cooked onion and herbs with a slotted spoon before serving.