In a large pot, bring the chicken broth and water to a simmer.
Add the lemongrass pieces, lime leaves (if using), galangal or ginger, garlic, and sliced chili. Let the aromatics infuse the broth for 5–7 minutes.
Stir in the fish sauce, lime juice, and chili paste (if using). Adjust heat and seasoning to taste.
Add the mushrooms and cherry tomatoes, and let them cook for 3–4 minutes until slightly softened.
Add the prawns to the pot and cook for 2–3 minutes, until they turn pink and opaque.
Remove from heat and discard the lemongrass pieces and lime leaves.
Stir in chopped cilantro and scallions (if using) just before serving for freshness.