In a pot, bring the broth to a boil over medium heat.
Add lemongrass, kaffir lime leaves, galangal, and Thai chilies. Simmer for 5 minutes to infuse flavors.
Add mushrooms and shrimp. Cook for 3-4 minutes until shrimp turns pink.
Stir in fish sauce, lime juice, sugar, and tomatoes. Simmer for 2 more minutes.
Taste and adjust seasoning (more lime for tang, fish sauce for saltiness, or chili for heat).
Remove lemongrass and galangal slices before serving.
Garnish with fresh cilantro.
For a creamy version, add 2 tbsp coconut milk at the end.