Melt the butter in a saucepan, add the onion and potato and cook gently, covered for about 10 minutes, stirring occasionally.Add salt, pepper and stock and bring to the boil. Simmer uncovered for about 5 minutes until the potatoes are tender. Add the sorrel leaves and cook for 5 minutes more. Liquidise the soup until smooth. To make the Cheese Croutons: Cut shapes from the bread and brush lightly with olive oil or butter. Grill on one side then turn over and sprinkle with the grated cheese and grill until cheese is melted.Place on top of the hot soup.