Heat oil in a large saucepan over medium-high heat, add onion and stir occasionally, until softened (5-6 minutes). Add curry powder and shrimp paste and stir until fragrant (20-30 seconds). Add chilli and lemongrass, stir until fragrant, then stir in coconut milk and fish sauce, season to taste and bring to the boil. Reduce heat to low, simmer until well flavoured (5 minutes), then add prawns and simmer until just cooked (4-5 minutes).
Scatter curry with spring onion and serve with rice.
