In a soup or stockpot, heat the coconut fat over medium heat. Add the onion and cook until softened, but not colored. Add the curry paste and cook for a minute or two, stirring constantly, then add chicken stock, the galangal or ginger, and the lemongrass stalks.
Simmer for about 10 to 15 minutes to infuse the stock with flavor, remove the chunks of ginger and the lemongrass stalks and discard. Add in the chicken, mushrooms, and sliced jalapeno or chile; cook at a low simmer until the chicken is just cooked through, five to six minutes.
Turn down to a bare simmer and add in coconut milk, then sugar, fish sauce, and lime juice. Simmer a minute or two to heat everything through. Taste and adjust flavors, adding a little more of any of the elements to achieve your perfect balance; sugar fo sweet, fish sauce for salty, and lime for sour. If you want more heat, consider stirring in more curry paste.
Serve with additional lime, sliced jalapeno or other chiles, cilantro and/or parsley, and maybe thinly sliced green onion.
Keywords: Asian, Chicken, Chicken Breast, Chicken Stock, chili paste, coconut milk, Fish Sauce, Hot Peppers, jalapenos, Lime, mushrooms, Soup, Thai.