Lightly salt the four pieces of fish and put to one side while you make the sauce.
In a large frying pan or shallow pan for which you have a lid, gently warm the olive oil and the chillies for a few minutes, until they sizzle.
Keeping the flame low, add the garlic, stir for a few seconds, then add the tomatoes, tomato concentrate and a pinch of salt.
Leave the sauce to come up to a lively simmer, then turn it down to a gentle one, cover and leave for 30-40 minutes, every now and then lifting the lid to stir.
Taste and adjust for salt, add a little sugar to sweeten, if you think it needs it, then stir in the olives, capers and parsley.
Lay the fish fillets in the sauce, making sure they are well spaced out and more or less immersed (they won’t be entirely covered).
Cover the pan and cook on a medium-low heat for 10 to 15 minutes (or bake uncovered in a 220C (200C fan)/425F/gas 7 oven for the same amount of time).
Serve topped with more chopped parsley.
