Heat the butter and oil in a
large saucepan and cook the onions and brown sugar
gently for about 20 minutes, stirring occasionally
until the onions start to turn golden brown.
Stir in the flour and cook for
a further 2 minutes. Pour in the consommé plus two
cans of water, then add the sherry and
Worcestershire sauce. Season with salt and pepper,
cover and simmer gently for a further 25-30 minutes.
Preheat the grill and, just
before serving, toast the bread lightly on both
sides. Spread one side of each slice with the
mustard and top with the grated cheese. Grill the
toasts until bubbling and golden.
Ladle the soup into bowls. Pop
two croutons on top of each bowl of soup and garnish
with chopped fresh parsley. Serve at once.