In a medium large saucepan place the butter and heat in on medium until it has melted. Add the mushrooms and onionsm and saute them for 3 to 5 minutes, or until they are tender. Sprinkle in the flour and stir it for 2 to 3 minutes, making certain that no lumps are formed. Add the chicken stock, white wine, bay leaf, and nutmeg. Bring the mixture to a boil and then simmer it for 15 minutes. Remove the bay leaf. Add the cornstarch-water mixture and stir it in well. Simmer the soup for another 10 minutes. Add the warmed heavy cream and stir it in. Season the soup with the salt and white pepper. Garnish the soup with the chopped parsley.