Mexican
Put the beans in a large pot with the water, half the onion in one piece, the garlic and the bay leaf; bring to a boil, then cover partly and simmer for 2-2.5 hours until very soft, adding hot water if the level drops below the beans.
Remove the onion and bay leaf, then drain the beans, keeping 250 ml of the cooking broth (the rest can go into a soup).
Finely chop the remaining onion. Heat the lard in a wide heavy frying pan over medium heat and fry the onion for 5 minutes until golden.
Add the beans a ladle at a time with the salt, mashing them into the fat with a bean masher or potato masher.
Add the reserved broth little by little and keep mashing and stirring for 10-15 minutes, until the beans form a thick paste that pulls away from the pan.
Taste for salt and serve hot as a side with eggs, tacos or rice, or spread on tortas and tostadas.