Put the black and yellow mustard seeds into a grinder and grind together. Empty the ground spices into a small bowl and add the turmeric, cayenne, salt and 1/2 cup water. Mix together.
Heat the oil in a frying pan over a medium flame. Add the nigella followed a second later by the red chillies. Stir once and add prawns. Stir for a minute until the prawns start to turn pink.
Add the mixed spice paste and the green chillies. Stir on a medium heat for 1-2 minutes until the sauce is a bit thicker.