Optional toppings: thinly sliced radishes, finely shredded cabbage, minced fresh chili peppers and sour cream
Rinse beans. In a Dutch oven, combine beans and water. Bring to a boil. Reduce heat; cover and simmer until beans wrinkle and crack, about 1-1/2 hours. Add onion, pork, salt, garlic and oregano. Simmer, covered, 1-1/2 to 2 hours, or until beans and pork are tender. Stir in tomato paste; heat through. Ladle into soup bowls. If desired, top with radishes, cabbage, peppers and sour cream.