In a large pot, bring the chicken 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 5-7 minutes to infuse the flavors.
Add the sliced onion and mushrooms to the pot, and continue to simmer for another 3-4 minutes.
Stir in the shrimp and tomato wedges, cooking until the shrimp turns pink and opaque, about 2-3 minutes.
Remove the pot from heat and stir in the fish sauce and lime juice. Adjust seasoning to taste.
Serve hot, garnished with fresh cilantro and optional green onions.