Place the chicken carcass into a large pot and cover with water. Bring to the boil over high heat, then reduce heat to medium-low and simmer for 5 minutes. Drain and rinse the carcass under running water. Return the carcass to the pot along with the galangal, lime leaves, lemon grass, chillies, garlic and onions. Pour in 2.5 litres of water.
Return to the boil over high heat, then reduce heat to medium-low. Simmer uncovered for 1 1/2 hours, skimming the foam and fat often. Pour through cheesecloth before using.