Shell and devein prawns. Heat oil in a large heavy-based saucepan over medium heat; add brown onion. Cook, stirring, until soft but not coloured. Add the paste; cook, stirring, for 1 minute. Add the curry powder, sugar and flour; cook, stirring, for 1 minute.
Gradually stir in the milk. Stir constantly until mixture boils and thickens. Stir in the prawns and peas until heated through. Season to taste with salt and freshly ground black pepper. Serve with steamed rice, sprinkled with green onion.