Heat the oil in a frying pan over a medium-high heat. Add the onion, garlic, ginger and a pinch of salt; fry for 6-8 minutes until soft.
Stir in the nigella seeds and garam masala; fry for 1 minute more. Add the tomatoes and simmer everything gently for 4-5 minutes.
Stir in the tamarind paste and cooked prawns, then heat for a final 2 minutes. Meanwhile, heat the rice according to pack instructions. Serve with the curry and coriander leaves, if liked.