Melt the butter or margarine in a large saucepan and fry (saute) the
onions gently for about 20 minutes until golden brown. Add the bacon and
potatoes and fry until just browned. Stir in the stock, bring to the boil,
cover and simmer for about 15 minutes until the potatoes are soft. Season
Meanwhile, mix the cheese, mustard and herbs and season to taste. Spread
on the bread slices and toast under the grill (broiler) until the cheese
is melted and golden brown. Float the bread on the soup to serve.