For the mackerel:
To make the spice paste, heat a dry frying pan over a medium heat, then add the seeds and peppercorns and toast, shaking from time to time, for 2/3 minutes until the mustard seeds are popping and the spices are aromatic. (You may need to cover the frying pan to stop the mustard seeds jumping.)
Blend the toasted spices with the chilli to a smooth powder in a coffee grinder or with a pestle and mortar.
Combine the powder with salt and the brown sugar. Store in an airtight container until needed.
Half an hour before cooking the fish, lightly sprinkle both sides of each mackerel fillet with the spice mixture - about 2 tsp per fillet. Store any remaining spice mixture for future use.
For ease of turning, place the fillets in a metal hinged rack and cook on a hot barbecue for 1 or 2 minutes each side until lightly charred and firm to the touch. Serve with vegetable raita.
For the Vegetable Raita:
Combine all the ingredients. Season to taste, cover and chill until required.
To store, cover and refrigerate for up to 2 days.
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