Melt the butter in a large
saucepan. Add the onions and sugar, reduce
the heat to very low and cook the onions, stirring occasionally, for
20-30 minutes until they are soft and a really deep golden brown. Stir the flour into the onion
mixture and cook over a very low heat, stirring constantly, for
about 5 minutes. Add the beef stock and season to
taste with salt and pepper. Increase the heat and bring to the
boil, stirring constantly. Reduce the heat and simmer for
15-20 minutes. Taste the soup and adjust the
seasoning, if necessary. Meanwhile, toast the slices of
French bread lightly on both sides. Sprinkle with the grated Gruyere. Pour the soup into a hot tureen. Place a piece of toast in each
serving bowl and ladle the hot soup over the top.