Rinse beans and combine with the broth and stock base in a large stockpot. Simmer, covered, until beans are tender (about 2 hours). Saute onions in oil until golden in color; add garlic and cook for 1 minute. Drain the chilies and add to the onion mixture along with the cumin, oregano and cayenne. Mix well. Add onion mixture and chicken to the stockpot. Simmer 20-30 minutes until the beans are soft. Salt and pepper to taste. Add sour cream and Monterey Jack cheese; continue cooking until cheese melts. Serve with optional garnishes.