Special equipment: 2 very large pots
For the beans: Place the beans in large bowls or pots, cover with cold water and allow to soak overnight. Drain and rinse before proceeding. (Alternatively, add the beans to a few large pots and cover with hot water. Bring to a boil, and then boil for 2 minutes. Turn off the heat, cover the pots and allow the beans to sit for 1 hour. Drain the beans and rinse them with cold water before proceeding.)
Divide the soaked beans between 2 very large pots. Into each pot, add 3 quarts (12 cups) chicken broth, 2 tablespoons salt, 2 tablespoons pepper, half of the garlic and onions and 1 gallon (16 cups) water. Bring to a boil, reduce the heat to low, cover and simmer for 1 1/2 hours.
For the spice mixture: Heat the oil in a skillet, then add the oregano, cumin, coriander and chile powder and fry until the color deepens (do not let it brown), 30 seconds to a minute. Divide the spice mixture between the pots of beans, along with the salt (3 tablespoons salt in each pot) and stir together. Cover and continue simmering until the liquid level is to your liking, about another hour. Taste for seasoning and add more of what it needs.
This is a large-quantity recipe for feeding a crowd, but it can be scaled down quite easily.