BRING broth, lime juice, fish sauce, sugar, lemongrass, galangal and kaffir lime leaves to boil in large saucepan. Reduce heat to low; simmer 15 minutes. Remove galangal, lemongrass and kaffir lime leaves.
STIR in chili paste until well blended. Add chicken, mushrooms, onion and chilies; simmer 10 to 15 minutes or until chicken is cooked through. Stir in tomato.
LADLE into soup bowls. Garnish with cilantro.
THAI Kitchen Tip: If preparing with shrimp or tofu, simmer broth for 25 minutes before adding chile paste. Add shrimp or tofu with mushrooms; simmer 1 to 2 minutes or just until shrimp turn pink or tofu is heated through.