Heat the oven to 220C/fan 200C/gas Put the onions and tomatoes in a shallow roasting tin or dish. Season, add 4 tbsp of olive oil and toss together. Roast for 15 minutes. Wrap each piece of cod in a slice of Parma ham. Add the olives to the trayand sit the cod on top of the tomatoes. Keep roasting for another 15 minutes until the cod is cooked through and the Parma ham has crisped up. Scatter with parsley and serve with some crusty bread.