Heat the oil in a large nonstick frying pan and cook the onion and ginger over a gentle heat for 3–4 minutes, stirring regularly until soft, but not golden. Add the spices and cook for a further 1 minute. Add the chopped tomatoes and sugar and increase the heat slightly, continuing to cook for a further 5 minutes, stirring occasionally, until the tomatoes have reduced slightly and thickened.
Pour in the coconut milk and bring to the boil. Reduce the heat and simmer, uncovered, for 10 minutes until the sauce has reduced and thickened. Drain the prawns well, then add to the sauce with the peas and coriander and cook for a further 2–3 minutes until piping hot.
Serve the mild curry in warmed serving bowls, with either rice or naan breads to mop up the juices.