Put the onion, tomatoes, cumin, coriander, one tbsp of oil and a large pinch of salt and pepper into a food processor and blitz until smooth. Transfer the tomato mixture to a saucepan, place over a medium heat and cook for 6 minutes, stirring frequently, until the sauce is slightly reduced. Set aside.
Heat the remaining oil in a large non-stick frying pan over a medium–high heat. Season the mackerel fillets on both sides with salt and pepper and add the fish to the pan, skin-side down (you may need to do this in batches). Fry for 3 minutes, without moving the fillets, until the skin is crispy and golden brown. Flip over and cook for another 1–2 minutes, or until the fish is just cooked through. Add the lemon juice and allow it to sizzle for a few seconds.
Transfer the fillets to warmed plates, spoon the spicy sauce over the fish and serve immediately.
Notes
To remove the bones down the centre of each fillet you can either use tweezers, or try carefully slicing down either side of the bones using a sharp knife. Take extra care not to cut through the skin.