Drain the black beans and put them in a large saucepan. Add the red onion and pork fat, cover with 2 inches of water and bring to a boil. Simmer the black beans over low heat, stirring occasionally, until the beans are tender, about 1 1/2 hours.
Set a colander over a large heatproof bowl and drain the black beans. Discard the onion and pork. Return the black beans to the saucepan and add about half of the cooking liquid to moisten them. Season the beans with salt and pepper and serve.