Marinate the prawns with the chilli powder, garlic,turmeric, cumin, lemon rind and lemon juice and refrigerate for an hour
Heat the 1/4 cup oil and flash fry the prawns in little batches and set aside. Be careful not to let the oil burn as you are going to use it to make the sauce.
Once all the prawns are fried add another tablespoon or 2 of oil to the remaining oil, only if needed. Fry onion until golden brown.
Add ginger/garlic paste and saute for a minute, add curry leaf and chillies. Add chilli powder,cumin, coriander,turmeric and cook for a minute.
Add tomatoes , season with salt. Add sugar if required. Cover and simmer until sauce thickens and tomatoes are fully cooked.
If you're using cream add it now and heat through
Add prawns to the sauce, bring to the boil and turn off heat. Garnish with coriander.