Heat the butter and oil in a large pan and add the
onions and brown sugar.
Cook 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 vegetable
stock, then add the sherry and Worcestershire sauce.
Cover and simmer gently for 25 to 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 soup dishes and place 2
croutons on top of each. Serve at once.