In a heavy bottomed pot or dutch oven, saute whole chicken breasts over medium heat until nicely browned and just cooked. Remove from pot and set aside.
Add a little chicken stock and scrape up the fond in the pot, then add the rest of the stock, turn up the heat and bring to a boil.
Meanwhile, slice and mince the lower portion of the lemongrass stalk. Retain the upper stalk for the soup pot.
Add mushrooms, prepared lemongrass (including upper stalk pieces), kaffir lime leaves & galangal. Boil 5 minutes.
Turn heat down to medium. Add coconut milk, fish sauce, and Spiracha. Slice reserved chicken and return to pot with any juices. Stir well. Simmer gently 1-2 minutes.
Turn heat down to minimum. Add sugar, lime juice and stir.
Do a taste test. Look for a balance between spicy, sour, salty, and sweet flavors. Start with salty, adding more fish sauce if not salty enough (1 Tbsp. at a time). If too sour, add a little more sugar if you need it. If too spicy (hot), add a little more coconut milk. If not spicy enough, add more Spiracha. If not sour enough, or to brighten the flavours the next day, add more lime juice.
Ladle soup into serving bowls. Sprinkle a little fresh cilantro, basil, and spring onion over each bowl.
**Note: if serving this to guests- especially those not familiar with Thai food, be sure to explain about what to eat and not to eat in the soup. While it is all technically edible, most people don't eat the lime leaves, galangal or lemongrass tops. You could explain that only the broth, chicken, and mushrooms are eaten, or, you could cook the soup with the galangal, leaves & stalks in a cheesecloth bouquet. I also usually leave the lemongrass stalks quite long (3 inches or so) so they don't make their way into the bowls. Otherwise, my husband & I just pick around the rest, and my dear Dad eats the whole shebang. Enjoy!
**Another note: no, no, NO you really can't substitute ginger for galangal, or lemon zest for lemongrass, or lime juice/zest for kaffir lime leaves. The flavours are not really all that similar. Believe me, you really can find these ingredients near you, even if you don't live in a large Metropolis or somewhere with a large Asian population. Take a real good look though your ethnic aisle or market, and expect to see some variations is spelling and preparation. Do a google search before you go, and familiarize yourself with what they might look like and brands you might recognize. The lime leaves are often sold frozen or dried- frozen is best as the essential oils containing all that lovely aroma are vastly diminished in the dry variety. Even so, dry is OK, just double the amount and simmer a little longer. Same goes for galangal, I've seen it dried and powdered as well as fresh, and they will do in a pinch. Fish sauce is crazy easy to find, but try to get a brand made in Thailand as japanese & chinese varieties have a different flavour. And before you ask, yes, the fish sauce really should smell that bad- what do you expect, it's fermented fish juice! Beleive me, you won't notice the badness of it, just the goodness, I promise!
If you insist on using fresh chilies, use 1 for mild, 2 for medium, and 3 for spicy.
