Heat the oil in a wide, lidded frying pan, then soften the onion for 5 mins. Increase the heat a little, stir in the ginger, garlic, shrimp paste, chilli and lemongrass, and cook for 2 mins. Add the curry powder and sugar, and keep stirring. When the sugar starts
to melt and everything starts to clump together, add the coriander stems, coconut milk and 2 tbsp water, then bring to a simmer.