Place the oil in a pan over a low heat and add the onion. Cook for 6-8 minutes, adding the garlic to the pan for the last minute.
Add the tomatoes and the crushed chillies and simmer over a low heat for 5-10 minutes, until thickened. Add a pinch of sugar and season to taste. Take off the heat and stir in the parsley and the lemon zest and juice. Set aside.
Preheat the grill to high. Oil a large baking tray thoroughly.
Cut each mackerel fillet into 3 on the diagonal. Place skin side up on the oiled baking tray and cook under the grill for 3-4 minutes each side.
Meanwhile, toast the sourdough slices (you could also do this under the grill or in a toaster).
To serve, spoon the tomato sauce over the toasted bread and top with the mackerel fillets. Garnish with extra parsley.
Cook's tip: You may need to add more or less sugar depending on the sweetness of your tomatoes. Taste as you go along and you can always add more if needed.