Scrub the courgettes and trim them. Now cut them into 4 slices lengthwise. Cut each slice lengthwise into 4 long strips. Cut the strips into thirds crosswise. You want them to be just a little larger than the prawns. Sprinkle over a quarter of a teaspoon salt and toss to mix. Set aside for 30-40 minutes then drain and pat dry.
Put the prawns on kitchen paper and dry them off as well.
Heat the oil in a wide saucepan or frying pan over a medium heat. When hot, put in the chopped garlic and stir and fry until medium brown being careful not to burn.
Add the courgettes, fresh coriander, green chilli, turmeric, cumin, cayenne, tomatoes and their liquid, ginger, lemon juice and remaining tsp salt. Stir to mix and bring to a simmer.
Add the prawns and stir them in then cover, turn heat to low and simmer for 3 minutes. Uncover, turn the heat to medium and reduce the liquid so you are left with a thick sauce.