Use a sharp knife to cut 3 slashes across each side of the fish. Mix together the olive oil, oregano and chilli and rub all over the fish. Season well, place in an ovenproof dish, scatter around the lemon slices and bake in a preheated oven, 220°C (425°F), Gas Mark 7, for 10 minutes.
Add the tomatoes and olives and return to the oven for 10–15 minutes until the fish is just cooked through. Serve with new potatoes roasted in olive oil and a green salad.