Grind together ginger, green chilli, coriander powder, chilli powder and turmeric powder with a little water. Combine prawns, mango & onion with ground masala. Add medium thick coconut milk & salt to this. Mix well. Cook till the gravy becomes thick & the prawns are done. Add thick coconut milk. Bring it to a boil and remove from fire.
Heat oil in a pan & splutter mustard. Add small onions & curry leaves. Fry it till the onion becomes golden brown. Add it to the curry. Serve hot with rice.
Notes
You can add or reduce the qty of raw mango, depending on its sourness. I use Maggi coconut milk powder. For 1 cup of thick coconut milk, add 3 heaped tbsp of coconut milk powder to 1 cup of water and for 1 cup of medium thick coconut milk, add 2 heaped tbsp of coconut milk powder to 1 cup of water. You can adjust the qty of coconut powder as per your requirements.