To make your sauce:
Peel and finely slice the garlic cloves. Pick the basil leaves and put aside, finely slice the stalks.
Add a good couple of lugs of olive oil to a large pan on medium heat. Add the garlic and basil stalks. Pierce the chilli once with a knife so it doesn’t explode when frying, and add to the pan. Fry gently until the garlic is soft but not coloured, stirring occasionally.
Add the tins of tomato and season lightly with the salt and pepper. Gently simmer on low heat for 30 minutes. Remove the chilli. Break and mush the tomatoes with a wooden spoon. Season the sauce really carefully with more salt and pepper, if needed, and add a tiny swig of red wine vinegar to give it a little twang.
To prepare and cook your fish:
Preheat the oven to 220°C/425°F/gas 7.
Pour your tomato sauce into a roasting tray (about 20cm x 30cm). Season the fish fillets on both sides with a little salt and pepper, then place on top of the sauce. Squash the olives, using the base of a jar or something heavy, and remove the stones. Sprinkle the olives and capers over the fish. Scatter the reserved basil leaves over the fish.
Cook in the oven for around 15 minutes or until the fish is cooked through (check by cutting into the thickest part of one or two of the fillets; they should be pearly white and not transparent). Lovely served with new potatoes and a green salad.