Heat 3 tablespoons of the oil in a flameproof casserole, add the onion, peppers and garlic and cook until soft and starting to brown. Add the tomatoes and cook for a few minutes until softened. Mix in the feta and oregano, season to taste with salt and pepper and drizzle with the remaining oil.
Cover and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes. Garnish with the parsley and serve with warmed crusty bread.