Peel the onions leaving them whole. Place in a saucepan, cover with cold water and bring to the boil. Simmer for 10 minutes then drain. Place in a casserole, season with salt and pepper and sprinkle over the sugar. Add the stock. Cover with a lid, place in a moderate oven (180°C., 350°F., Gas Mark 4) and bake for 25–30 minutes or until tender.
Remove the onions from the casserole and place in a hot serving dish. Pour the juices into a saucepan. Blend the cornflour with the vinegar, dilute with a little of the hot juice, then stir into the saucepan. Bring to the boil, stirring all the time until thickened and clear. Check the flavour and pour over the onions. Serve with grilled pork or lamb chops or with steak