Melt the butter in a
saucepan. Add the onion, celery, carrot and potato
and fry (sauté), stirring for 2 minutes.
Add the
stock, olives, bay leaf and pasta. Bring to the
boil, reduce the heat and simmer gently for 12-15
minutes until the vegetables and pasta are really
tender.
Season to taste and ladle into flameproof
bowls. Pile the cheese on the toasted bread.
Float a
piece on each bowl and flash under the grill
(broiler) until the cheese melts and bubbles. Serve
straight away.