Puree 2 cans of black beans in 2 cups water in food processor. Heat the lard or other fat in a large saucepan over medium heat. Saute the onions with the salt and cumin until golden, about 10 minutes. Add the beans and continue cooking over medium heat, stirring frequently, until the liquid evaporates and the beans form a creamy mass that pulls away from the bottom and sides of the pan, about 15 minutes. Serve immediately.