Put the meat in a large bowl and add the wine, onions, garlic and basil. Set aside to marinate at room temperature for 6 hours, basting occasionally. Remove the meat from the marinade and pat dry with kitchen towels. Reserve the marinade.

Preheat the oven to moderate 180°C (Gas Mark 4,350°F).

Melt the butter with the oil in a large flameproof casserole. Add the meat and fry until it is evenly browned. Add the marinade and bring to the boil. Add salt and pepper to taste and transfer the casserole to the oven. Roast for 2 hours. Add the tomatoes and roast for a further 1 hour, or until the meat is cooked through and tender. Remove from the oven and transfer the meat to a carving board. Cut into thick slices and arrange decoratively on a warmed serving platter. Keep hot while you make the sauce.

Strain the liquid into a saucepan, pressing down on the vegetables with the back of a wooden spoon to extract the juices. Bring to the boil, then add the olives. Reduce the heat to low and stir in the cornflour (cornstarch) mixture. Cook, stirring constantly, until the sauce thickens and is smooth.

Pour over the meat slices, sprinkle over the parsley and serve at once.