Heat the butter in a large, deep (preferably non-stick) sauté pan, over a medium heat, add the sliced fennel (NOT the strips of fennel), sliced leek and finely chopped garlic, and sauté, while stirring occasionally, for 3-4 minutes or until the fennel, leeks and garlic are beginning to soften.
Remove from the heat, and set aside.
In a large pot, heat the stock, add the rosemary sprigs, thyme and nutmeg and bring to the boil.
Simmer over a low heat for about 15 minutes.
Remove the rosemary sprigs.
Add the fennel, leek and garlic mixture to the pot, and stir to combine all the ingredients.
Set the pan in which the fennel, leek and garlic were sautéed aside, for re-use later.
Allow the soup to cool slightly (10 minutes cooling time should be ample), then blend it in batches in a blender or food processor until smooth.
As each batch is blended, pour the blended soup into another pot or pan, perhaps the deep pan used earlier to sauté the fennel, leeks and garlic.
Once all of the soup has been blended, stir it to ensure that it is of an even consistency (inevitably some batches were probably thinner than others).
Stir the sour cream and the 1/4 cup of parmesan into the soup.
Reheat the soup over a medium heat until it is hot, but do not allow it to come to the boil.
Season to taste with salt and freshly ground black pepper.
Turn off the heat and leave the soup covered so that it stays warm.
In a small non-stick pan, heat the oil, and sauté the extra leek for 2-3 minutes, or until softened but not browned.
Ladle the soup into warm soup bowls and top with the sautéed leek.
Garnish with the extra parmesan and sour cream, and serve immediately with warm crusty rolls.
Chef's Note: Since posting this recipe, and since last making this soup, I have found and posted a fabulous recipe for vegetable, quite unlike others I have used in the past: Vegetable Stock Vegetable Stock .