How to Make It
Drain the black beans, then, in a large enameled cast-iron casserole, combine them with enough water to cover by 3 inches. Add the bay leaf, thyme, parsley, quartered onion and 1 tablespoon of salt. Bring to a boil over moderately high heat. Reduce the heat to low, cover and cook until the beans are just tender, about 1 hour. Drain the beans and discard the bay leaf, thyme, parsley and onion. Wipe out the casserole.
In the casserole, heat the olive oil. Add the chorizo and cook over moderate heat, stirring occasionally, until starting to crisp, about 4 minutes. Add the chopped onions, the garlic and cumin; season with salt and pepper. Continue cooking over moderate heat, stirring occasionally, until the onions are tender, 10 to 12 minutes. Add the stock and 2 cups of water and bring to a boil. Return the beans to the casserole, cover, reduce the heat to low and continue cooking until the beans are very tender and the soup has thickened, about 1 hour and 45 minutes.
Ladle the soup into bowls and serve garnished with crema, avocado, radishes and cilantro. Pass lime wedges and warm tortillas at the table.