Dust the trout with flour. Heat the olive oil in a frying pan and brown the trout for 2–3 minutes on each side. Place the fish in an ovenproof dish big enough for them to sit in a single layer.
Add the onion and olives to the frying pan, sauté for 3–4 minutes and then spoon over the fish. Scatter over the chopped tomatoes, lemon juice and capers and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 15–18 minutes, until the fish is cooked through.