The red chilies are also known as "prik khee nu". Puree peppercorns and coriander roots. Trim root and tough layers from lemon grass. Thinly slice first six inches. Bring 2 c. of stock to boil. Add coriander paste and simmer while adding lemon grass, ginger and shrimp. Stir in remaining stock and bring to boil. Season with lime juice, fish sauce and chilies. Sprinkle servings with chopped coriander.