Cut the calf's foot into four pieces. Cut the tripe into small squares. Put them into the pan with the onion, garlic, peppercorns, salt, and water. Cover and bring to a boil. Lower the flame, open the windows(!) and simmer uncovered for about 2 hours, or until the tripe and foot are just tender but not too soft.
Meanwhile, toast the chiles anchos well on a dry skillet, turning them from time to time until the skin is charred. Slit them open (wearing rubber gloves, if you like keeping your skin) and remove the seeds and veins. Grind them dry to a fine powder. Add it to the pot as the meat is cooking. Remove the pieces of the calf's foot from the pan, and when they are cool enough to handle, strip off the fleshy parts. Chop them roughly and return them to the pan. Add the hominy and continue cooking the menudo slowly, still uncovered, for another two hours. Adjust the salt to taste. Sprinkle with oregano and serve.