Put the onion, red peppers and garlic cloves into a roasting sheet pan and toss them with 2 tablespoons of olive oil. Roast in the preheated oven for 15 minutes, then mix up the vegetables and return the tin to the oven to roast for a further 10-15 minutes or until soft and slightly golden.
Pour the tomatoes over the vegetables, then mix in the dried herbs and olives and season well. Roast in the oven for 5 minutes.
Lay the fish fillets on top of the vegetables, gently pushing them down into the tomato sauce. Drizzle the remaining olive oil onto the fish and season the fillets on top. Roast for 12-15 minutes or until the fish is cooked through.
Scatter the chopped parsley over the fish. Serve with lemon wedges to squeeze over and some roasted potatoes or crusty bread alongside if you’d like.