In a kettle cook the onion and the leeks in the butter over moderate heat, stirring, for 10 minutes, or until they are softened. Add the mushrooms and cook the mixture stirring, for 10 minutes. Add the wine, the bay leaves, the stock, the salt, and the white pepper, simmer the mixture for 30 minutes and discard the bay leaves. In a blender or food processor blend the soup in batches until it is smooth and return it to the kettle. Add the cream, heat the soup over moderately low heat until it is hot, and add salt and white pepper to taste. Makes about 12 cups soup 10