Lightly salt the four pieces of fish and set aside while you prepare the sauce.
In a large frying pan or shallow pan with a lid, gently warm the olive oil and chillies for a few minutes until they begin to sizzle.
Keeping the heat low, add the garlic, stir for a few seconds, then add the capers and olives. Stir again for a few seconds, then pour in the passata.
Lay the fish fillets in the sauce, making sure they are well spaced and mostly submerged (they do not need to be completely covered).
Cover the pan and cook over medium-low heat for 10 to 15 minutes (or bake uncovered in an oven preheated to 220°C / 200°C fan / 425°F / gas mark 7 for the same amount of time).