In a small sauce pan simmer the lemon grass and laos root slices in 1 cup of stock for 1/2 hour. Add more liquid as necessary to retain 1 cup. meanwhile in lg sauce pan simmer coconut milk and remaining 2 cups of stock, uncovered minutes ( covering may cause curdling). Add chilies scallions, cilantro, and the fish. strain lemon grass and laos root and add liquid to the pot. Simmer uncovered until fish is just cooked. remove from heat stir in lime juice and fish sauce.