Remove the outermost leaf from each stalk of lemongrass. Pound stalks to release flavor; cut into 1-inch lengths. Add the lemongrass, Thai chiles, garlic, lime leaves, and galangal to the broth. Cover and boil for 10 minutes.
Reduce heat and add shrimp, mushrooms, onions, and tomatoes to the broth. Boil for 3 minutes. Add fish sauce and sugar; cook for 2 minutes more and remove from heat. Stir in cilantro and lime juice.
