Remove any membrane from the fish, then divide the fish into four equal portions.
Mix together the yoghurt, lemon juice, cayenne pepper, garam masala, garlic, ginger and paprika, then season to taste. Rub the spicy dahi into the fish, cover and chill for 1 hour 30 minutes.
To make the relish, mix the chilli, garam masala, lemon juice and mint together and add salt to taste. Cover and chill until required.
Lightly oil a heavy-based frying pan and set it over a medium-high heat. Remove the fish from the spicy dahi and, using your fingers, gently wipe off the excess marinade. Cook for about 10 minutes, turning the fish regularly so that it cooks evenly. Serve hot with the mint relish.
Variation: halve the quantities and serve as a starter with cucumber, onion and tomato.