METHOD
1. Bring the water or stock, fish sauce, lime leaves, lemongrass, galangal, garlic, salt and pepper to a boil in a large pot over medium flame. Reduce heat to low and simmer for 15 to 20 minutes. Remove from heat and set aside for another 10 minutes to steep. Strain through a fine-meshed sieve and discard solids.
2. Return the stock to a simmer over medium-low heat. Add the shrimp, scallions and mushrooms and continue to simmer over medium-low heat until shrimp is cooked through, 5 to 6 minutes.
3. Stir in the lime juice, cilantro and chilies, adjust seasoning and serve.