Directions

1. 1. Place the chicken stock in a heavy-bottomed pot over high heat. Add the lime zest, lemon zest, ginger, green chilies, and the cilantro stems and roots. Bring to a boil, reduce heat, and simmer until reduced to about 3 cups. Press occasionally on the solids with a wooden spoon to release flavor.
2. 2. Cut the chicken breasts into broad slices, as thin as possible. Salt lightly, and reserve in refrigerator.
3. 3. When broth is reduced, strain into a measuring cup. Press on the solids in the strainer with a wooden spoon. Place the coconut milk in the soup pot over moderate heat. Slowly stir in the reduced broth. Cook over moderately high heat, but do not allow to boil (otherwise the coconut milk will separate). When the soup is just below the boiling point, add the chicken slices. Remove from heat after 30 seconds (or when the chicken is just cooked through).
4. 4. Just before serving, mix together the lime juice and sugar in a small bowl. Blend in the fish sauce. Pour mixture into hot soup, season to taste, and garnish soup with a little bit of chopped scal lion and cilantro leaves.
5. 5. Optional step: If your soup has curdled a bit or if you'd like a fluffier texture in the soup, place the soup in a blender at high speed for 30 seconds before you add the chicken.