Place the prawns into a bowl, sprinkle over the salt, ground turmeric and red chile powder, and set aside to marinate for 10 to 15 minutes.
Meanwhile, blend the English mustard, cornflour and a splash of water, to a paste in a small bowl. Reserve.
Heat the mustard oil in a large nonstick pan over a medium heat until smoking. Remove from the heat and set aside to cool for 15 to 20 seconds, then add the nigella seeds, return the pan to the heat, and cook the seeds until they sizzle.
Add the green chiles, and onion, stir well and fry for 7 to 8 minutes, or until softened.
Add the garlic, and ginger pastes, and continue to cook for 45 to 60 seconds. Add the reserved mustard paste, and grated coconut, and stir well to combine. Fry the mixture for 4 to 5 minutes, then add the water, and continue cooking for another 3 to 4 minutes, or until the volume of liquid has reduced by half, and the sauce has thickened.
Add the prawns and cook for 2 to 3 minutes, or until pink, stirring well. Stir in the chopped cilantro, and serve immediately.
*Can be found at specialty Asian markets.