A staple Mexican recipe for beans, served as a side to innumerable dishes. The type of beans used for this varies by region: pinto beans in the north, Peruvian beans are common in Central Mexico, and black beans in the south. It is traditionally prepared in an earthenware pot, which does impart a different flavor to the beans.
Place beans in a large, heavy pot. Add enough water to cover by about 3 inches. Add the onion and bring to a boil.
Simmer, partially covered, until beans are soft.
Add the salt and continue to simmer until beans are fully cooked and the cooking liquid is a thick, soupy consistency.