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 15-20 minutes. Remove from heat and set aside for another 10 minutes to steep. Discard solids.
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, 5-6 minutes.
Stir in the lime juice, cilantro and chilies, adjust seasoning and serve.
Variation: Add the shrimp shells from peeling the shrimp to the simmering stock for extra flavor.
Variation: Sliced bamboo shoots can be added if you like.
Variation: If kaffir lime leaves are unavailable, zest the limes before juicing them and use the zest instead.
