First make the masala paste. Tip the seeds into a dry frying pan, toast for 1-2 mins until aromatic, then tip into the small bowl of a food processor. Add the remaining paste ingredients, season with salt and whizz until everything is finely chopped and well combined. Add a splash of water and combine well.
Slash each mackerel a few times on each side. Rub the paste all over each fish, making sure you get into the slashes. Cover with cling film and chill for 1 hr, or overnight if you like.
For the yogurt, combine the ingredients in the small bowl of your food processor, season well and chill until ready to serve. Put the ingredients for the pickled onions in a bowl. Massage the lime juice and sugar into the onion rings for 1-2 mins, then cover and set aside at room temperature to pickle for 30 mins, or overnight if you prefer.
If serving with rice, make sure you have this on the go. Heat the grill to high. Put the mackerel on a baking tray lined with foil. Drizzle with a little oil and grill for 5-6 mins each side, or until the flesh is cooked through – it should easily slide away from the bones when pushed with a knife.
To serve, arrange the fish on a platter. If the yogurt has thickened, loosen it with a drizzle of water – it should be like single cream. Drizzle it over the fish, then top with pickled onions, coriander leaves and lime wedges. Serve the rice on the side, with extra green yogurt sauce.