Cut the onions into thin rings, then halve these so they are easier to eat. Crush the garlic clove. Heat the margarine in a saucepan, then add the onions and garlic and cook steadily for about 10 minutes, or until the onions are golden in colour. Stir well so these do not burn. Add the flour, blend with the onions then pour in the stock or water. Bring the liquid to the boil add the stock cube, if using this a little seasoning and the sugar. Cover the pan and simmer gently for 20 to 25 minutes, or until the onions are tender. Spoon into individual soup bowls, top with the bread or toast and cheese. Serve at once or place under the grill for 1 to 2 minutes only to melt the cheese. Do not leave for too long a period, otherwise the cheese becomes ` stringy'.