Place the prawns in a medium bowl and add the lemon juice, chilli powder, salt and black pepper. Stir to coat the prawns, cover and leave to marinate for about 20 minutes.
Spray a large frying pan with low-calorie cooking spray and place over a medium high heat. When the pan is hot, add the prawns, discarding the marinating juices. Sear the prawns for about a minute on each side, then place on a plate and set aside.
Lower the heat to medium and add the onion and red pepper to the frying pan. Cook for 5-10 minutes, stirring, until softened.
Reduce the heat to low, add the garlic, ginger, garam masala, cumin, ground coriander, turmeric and paprika and cook gently for 1-2 minutes, to release the aromatic flavours from the spices.
Add the tomato puree, tomatoes and sweetener or sugar and stir. Cover and simmer gently for 5-10 minutes, until you have a thick sauce.
Add the seared prawns and stir in. Cover and simmer gently for 2-3 minutes, just until the prawns are cooked through and piping hot. Take care not to overcook the prawns as they will become tough.
Remove from the heat and stir in the yoghurt and coriander leaves until completely combined. Serve with a small portion of Basmati rice or other accompaniment of your choice.