Melt butter in large saucepan, add the onion, leek, carrot, and celery. Cook stirring until the vegetables are softened about 10 minutes. Add the beef broth, tomatoes, potato, parsley and bay leaf. Bring to a boil and reduce the heat to low. Cover and simmer until the vegetables are very soft 40 to 50 minutes. Remove from heat and allow the soup to cool slightly. Discard the bay leaf, then puree the soup in batches in a blender or food processor. Return the pureed soup to the saucepan and taste for seasoning. Add a little sugar if necessary. Add salt and pepper. While the soup is simmering , mix meat, parsley, nutmeg, salt and pepper. Form into 20 meatballs. Bring a pot of water to boil. Drop the meatballs into the boiling water and poach until they float to the surface 2 to 3 minutes. Remove the meatballs with a slotted spoon and set aside. When read to serve, heat the soup but do not let it boil. Whisk in the cream if using. Add the meatballs to the soup just before serving.