Place prawns in a bowl. Add the yoghurt and sprinkle on 1/2tsp salt, 1/2 tsp turmeric and 1/4 tsp chilli powder and mix well. Keep aside
Heat 4tbsp oil in the pan. Once hot, add the mustard seeds, cumin seeds, 1/2 of the curry leaves and onions. (The spices should sizzle as soon as they go in the pan).
Mix well and add ¼ tsp salt. Cook the onions for for 3-4 minutes until translucent.
Now add the ginger and garlic and cook for 2-3 minutes. (If you feel your pan is looking dry or your ingredients are catching, add a little more oil or add a little water).
Once the onions, ginger and garlic look light golden in colour, add the chopped tomato and chilli. Finely chop the coriander stems and add them to the pan too.
Now place the tomato puree in a small bowl and about 1/4 cup (60ml) of water. Mix well to form a watery paste, then add this to the pan.
Continue to cook on medium heat - you will see the masala paste thicken. When you notice the masala paste release its oil around the edges, lower the heat.
Now add 1/4 - 1/2tsp salt (as per your taste), paprika, coriander powder and sugar and continue to cook for 30 seconds. (Again, if you feel the spices are catching, add a little water)
Now add the marinated prawns and increase the heat to medium/high. Mix well and cook for a minute. At this point add 1/2 cup (120ml) of water as well as the remaining curry leaves. Mix again and place a lid on top. After a couple of minutes, mix once again and allow to cook for a further 2 minutes with the lid on.
You will see the sauce will have thickened and should look nice and glossy. Once the prawns are completely cooked through, Taste for seasoning and adjust accordingly.
Turn the heat off and serve with steamed basmati rice, naan or chapatis.