In a large saucepan, heat the oil then add the chopped onions, ginger, garlic and cardamom and fry gently for about 10 minutes till softened and light-golden brown.
Then add in the spices and stir and cook for another 10 minutes (you may pour in a little water if you find the spices sticking to the bottom of the pan).
Add in the prawns and stir them so they are evenly coated with the spice mixture then add in the chopped tomatoes and season with salt and bring it to a simmering point.
Then reduce heat to medium-low and cook for about 20-25 minutes uncovered, stirring occasionally, by which time most of the liquid will have evaporated and the tomatoes will have reduced to a thick sauce.
Remove from heat and garnish with chopped coriander and serve straight away.