Leaving the shells on, slit the back of the prawns and devein. Rinse and put in a bowl with turmeric, coriander, fennel, cumin, chilli powder, salt and pepper. Mix and leave to marinate for a few minutes.
In mortar and pestle (or blender) pound the garlic and shallots until fine.
Heat a little oil in pan until hot. Add the curry leaves and stir-fry for about 2 minutes. Add the pounded garlic and shallots and stir-fry until fragrant. Put in the prawns, stir and cook until just done. Taste for seasoning and serve hot.
Instead of measuring the individual spices, you may also use pre-packaged fish curry powder that's available.