In small bowl combine 2 teaspoons of fish sauce, ginger, and turmeric. rub the fish on both sides w/ this mixture and let stand for 5 min. In a non-reactive sauce pan, bring 5 cups water and lemon grass to a simmer over medium heat and then simmer the fish for c. 4 min or till just cooked through. transfer the fish to a plate to cool slightly. remove the skin and separate each peice into 2 peices (4 total). bring the soup back to a simmer and add all remaining ingredients (except the fish). and simmer for 3 min. set a piece of fish in each bowl and ladle hot soup over fish. serve w/ jasmine rice.