Heat butter in a heavy sauce pot over moderately low heat. Add onions without letting them brown. Add parsnips and apples, stirring to mix. Add stock and bring to a boil. Simmer until the parsnips and apples are tender, about 30 minutes. Puree wth a hand blender and strain. Discard solids. Pour smooth soup into a clean sauce pot and bring to a boil. If the soup is too thick add a little stock. Season to taste with salt and white pepper. Whip the cream until lightly thickened but not stiff. Do not overwhip. Mix the Calvados Apple Brandy into the cream. This further mixing should thicken the cream so that it holds soft peaks. Just prior to serving, garnish each portion of the soup with a spoonful of cream. Pass under the broiler very briefly to lightly brown the cream. Serve immediately.