Peel the onions and push a single clove into the base of each. Place in a greased oven-proof dish, spoon over the beef stock and then season with salt and freshly-ground pepper to taste. Gently heat the butter until it just melts and then spoon it over the onions.
Bake in a preheated moderate oven (Mk 3 - 325ºF - 170ºC) for 75-90 minutes until tender. Place a sugar cube on top of each onion and sprinkle with caster sugar (superfine sugar). Cook under a pre-heated hot grill until the sugar is completely melted (caramelised). Serve hot with roast or grilled meat.
If you don't have the time or inclination to make your own beef stock, you can substitute a beef stock cube dissolved in 5fl oz (150ml - ¼pt) hot water.