Tom ka gai soup, with its creamy coconut base and tangy flavor, is one of my top comfort foods. We made it in my cooking class in Chiang Mai last fall. It's quite quick and easy, but does require a couple ingredients hard to find in my part of the world - kaffir lime leaf and galangal. (I have both frozen, which I ordered fresh from www.templeofthai.com.)
Note that the galangal, lemongrass segments, chillis, and kaffir lime leaf are not meant to eaten, but used to flavor the broth. You can keep these in for presentation, especially the chillis, but restaurants tend to strain them out.
Adapted from Sompon Nabnian's Chiang Mai Thai Cookery School cookbook:
1. Cut the bottom third of a lemongrass stalk into 1-inch segments. Slice four thin coins off a piece of galangal. Tear two kaffir lime leaves into pieces. Put all these in a wok with 1 1/2 cups coconut milk, 1 1/2 cups water, and 4 to 5 red thai chillis. Bring to a boil, simmer 10 minutes and strain, if desired.
2. Return the liquid to the wok and add 6 ounces thinly sliced raw chicken breast, a cup of straw mushrooms, and 4 thinly sliced shallots. Season with fish sauce and bring back to a boil. Add a handful of chopped cilantro and turn off the heat. Stir in the juice of a lime. Serve garnished with thinly sliced scallions and extra cilantro.