If you're an aficianado of Thai cuisine, no doubt you've encountered Tom Kha Gai soup. One thing that really stands out in Thai dishes is the complexity of flavors, a trait that's perfectly exemplified in this very simple recipe. The soup is layered with flavors — each item contributing its heat or salt or mellowness — to create a complex, intriguing whole. Take away any one of the elements and the soup is somehow flat and lacks the perfect balance of the whole.
Kaffir lime leaves are dark, glossy green leaves with a unique shape where 2 leaves are joined end to end. They have floral-citrus aroma and can be found either fresh or dried in Asian markets or at markets like Whole Foods. If you buy fresh leaves, you can freeze them for future use. Not all Asian fish sauce is created equal, so if you have access to a well-stocked Asian market, look for Three Crabs brand, which has a very light, fresh taste.
Combine the chicken stock, coconut milk, lemongrass, ginger, jalapeño and Kaffir lime leaves, if using, in a large saucepan. Bring to a boil over high heat, then reduce the heat to low and simmer the mixture for 15 minutes.
Add the mushrooms to the saucepan and cook over medium heat for 5 minutes, or until they start to soften. Add the chicken and cook until the chicken is just cooked through, 3 to 5 minutes.
Remove the pan from the heat and add the sugar, fish sauce and lime juice. Discard the Kaffir leaves and season the soup with salt and pepper, if needed. Divide soup among 4 large bowls and garnish each serving with a sprinkling of fresh herbs. Serve hot.