In a covered deep pot over medium heat, bring the water, chicken pieces, onion, peppercorns and salt to a boil. Reduce heat to medium low and simmer for 25-30 minutes or until chicken is almost tender. Add potatoes, corn and fish sauce and cook for 10-12 minutes. Add bok choy and continue cooking for another 5-7 minutes or until vegetables are tender. Correct seasonings to taste. Remove fat that may accumulate on top of the soup. Serves immediately with steamed rice.