In a large pot, bring the chicken or vegetable broth to a boil over medium-high heat.
Add the lemongrass, kaffir lime leaves, galangal, and bird's eye chilies to the pot. Let it simmer for about 5-10 minutes to infuse the flavors.
Add the sliced onion and mushrooms to the pot and cook for another 3-5 minutes until the mushrooms are tender.
Stir in the shrimp and tomato wedges, cooking until the shrimp turn pink and opaque, about 2-3 minutes.
Season the soup with fish sauce, lime juice, and sugar. Taste and adjust seasoning as needed.
Remove from heat and discard the lemongrass, galangal, and lime leaves if desired.
Serve hot, garnished with fresh cilantro and lime wedges on the side.