Heat the oil in a medium saucepan. Tip in the onion and ginger, then cook for a few mins until softened. Stir in the curry paste, then cook for 1 min more. Pour over the chopped tomatoes and coconut cream. Bring to the boil, then leave to simmer for 5 mins, adding a little boiling water if the mixture gets too thick.
Tip in the prawns, then cook for 5-10 mins more, depending on how large they are. Serve alongside some plain rice and sprinkle with a little chopped coriander, if you like.