Boil water in a small stockpot and add the chicken, lemongrass, and garlic. Return to a boil and skim the surface thoroughly. Reduce heat to a simmer, partly covered, until the chicken is tender, 25 to 30 minutes. Add the fish sauce, salt and sugar.

Remove the chicken and set aside on a plate. Cool until you can shred the meat from the bones with your fingers. Return the meat to the pot.

Remove the lemongrass from the broth and discard. Stir in the lime juice, scallions and basil. Serve immediately with sliced chilies and more fish sauce on the side.

Note that Cambodians fish out all of the solids and eat them "dry" with rice and more fish sauce and then eat the broth separately.