Dry fry cumin seeds in a pan over medium heat for three minutes until dark brown.
Mix yoghurt, water, turmeric, chilli powder, ground coriander, ground cumin, salt, and sugar in a bowl.
Add the yoghurt mixture to the toasted cumin seeds and bring to a boil.
Simmer the sauce for three minutes before adding whole chillies and par-boiled potato cubes.
Simmer for 8 to 10 minutes until the potatoes are cooked through.
Season mullet fillets with salt and pepper.
Heat oil in a frying pan and cook mullet for two minutes on each side.
Add butter to the pan and baste the fish with the foaming juices until cooked.
Plate potatoes and sauce first, then top with a mullet fillet and garnish with coriander and lime.