Heat oil in a pot and fry lemongrass, lime leaves, half the lime rind, chilli, ginger, coriander root and green onion for about 4 minutes, or until softened. Add sambal bajak and fry for another minute. Add stock and coconut milk and bring to the boil. Reduce heat and simmer for 20 minutes.
Strain soup through a fine mesh sieve, discarding solids. Return soup t