Mix tamarind with ½ cup water and set aside. Take all roasting ingredients in a dry pan and roast until aromatic. It will take around 3 to 4 minutes. Allow the spices to cool, take it in a blender and powder coarse. Set this aside.
Heat oil in a large kadai. Add in fenugreek seeds, mustard, cumin, garlic, dry red chillies, curry leaves and fry them for 2 mins. Add in chopped onions and green chillies. Season with salt and turmeric powder. Saute this for 5 mins or so till it is cooked.
Now add in ginger garlic paste and saute till raw smell leaves. Add coriander and chilli powder and mix well. Cook the spices for 2 to 3 mins till the raw smell leaves from it. Add in tomato puree and cook that till it softens and gets mushed up.
Pour in tamarind water and salt, cook until thickened and oil separates.
At this point add in prawns and mix well. Cover and Cook the prawns for 6 to 8 mins in the masala. Now the prawns are cooked, sprinkle the powdered spices, jaggery, garam masala powder and coriander leaves. Serve.