Skewer the mackerel fillets with wooden or metal skewers (soak wooden ones for 30 minutes before use - it helps to stop them scorching).
Place in a shallow non-metallic bowl. Spread with the curry paste then squeeze the lime juice over the fillets.
Sprinkle with the chopped chilli and coriander and leave for 10 minutes to marinate.
Meanwhile heat up the grill and make the salsa - place the cucumber in a bowl with the tomato, onion and cumin seeds, stir in the chopped mint, then season to taste.
Cover with clingfilm and chill until needed (it will keep in the fridge for two days).
Grill the mackerel for 3-4 minutes until the fish flakes easily. Serve with the salsa, salad leaves and lime.