Remove bones and skin from chicken thighs; cut chicken into 1-inch pieces. Mix lemon grass and 1 Tbsp. Fish sauce in glass or plastic bowl; stir in chicken. Cover and refrigerate at least 1 hour.

Heat oil in wok or 10-inch skillet until hot. Add sliced onion and garlic; stir-fry 1 minute. Add chicken and scallions; stir-fry 5 minutes. Reduce heat; cover and cook, stirring occasionally, 2 minutes.

Mix vinegar and ginger root; reserve. Mix remaining ingredients except rice; stir into chicken mixture. Stir in reserved vinegar mixture. Heat to boiling, stirring constantly; cook and stir until thickened, about 1 min. Serve with rice.