Heat the oil in a karai or wok over a high heat. When it is hot lower the heat to medium, add the nigella seeds and stir for a few seconds then add the potatoes.
Stir fry for 7-8 minutes or until the potatoes start to brown at the edges.
Add the onions, turmeric, chilli powder and salt and stir fry for a few more minutes. Add the water and cook over a medium high heat until the liquid has reduced by half.
Add the shelled peas and green chillies and continue to cook until the water has been absorbed. Add the prawns and stir fry until the prawns are cooked.