Make the curry paste by placing all the ingredients in a food processor with 1 to 2 tablespoons of water and blending until smooth.
Heat a large wok or frying pan and the spray with low-fat cooking spray; add the mustard seeds and fry until they pop and release their aroma. Then add the curry sauce paste and fry for a further 3 to 4 minutes before adding 250 mls (9 fl oz) of water.
Stir in the tinned chopped tomatoes and cook for a further 5 minutes. Then add the prawns and simmer gently for 4 or 5 minutes, or until they are firm and pink in colour.
Check and adjust the seasoning if needed and then add the yoghurt and chopped coriander, gently stirring it in whilst the pan is off the heat.
Serve immediately with steamed (or boiled) Basmati rice; a 60g serving is 95 calories. An average naan bread is about 350 calories and a mini Weight Watchers naan bread is between 98 and 107 calories, depending on flavour.