Put the turmeric, cumin, coriander, chilli powder,mace, garlic and ginger into a bowl. Add a little water to form a paste then set aside.
Heat the oil in a large saucepan and add the onion and a little sea salt. Fry gently until it softens then add the spice paste and cook, stirring, for about a minute.
Add the chopped tomatoes then fill the empty can with water and add this to the pan. Squeeze in the lemon juice and add the cinnamon stick and curry leaves. Simmer gently for about 30 minutes to thicken.The sauce can be prepared in advance and left for a few hours.
When you are ready to eat add the prawns to the sauce and let them cook through but be careful not to overcook.
Pour the curry into a shallow bowl and grate over the fresh nutmeg and lemon zest.Sprinle over the coriander. Serve with basmati rice.