Heat a little oil in a medium pan and add the seeds except for the black onion seeds and gently fry, followed by the garlic and chilli, until the garlic turns golden.
Add the leeks and cook until they start to soften and add the curry powder. Add the coconut milk and boil until slightly reduced, before adding the tomato purée and black onion seeds. Cook until the leeks are soft and saucy, season and add the fresh coriander.
Heat a heavy-based frying pan until it starts to smoke. Season the bass, add a little oil to the pan and add the fish skin side down. Once the skin has crisped, turn the fish over, add the butter and remove from the heat. Set aside for 3-4 mins as the residual heat in the pan will complete the cooking.
Serve with the leeks, watercress and any left-over butter.