- In a large pot, soften the leeks and celeriac in the butter. Add the broth and bring to a boil. Simmer, uncovered, for 20 minutes or until the vegetables are tender.
- In a blender, purée the soup until smooth. Strain through a sieve. Season with salt and pepper. Add the cream, if desired. Keep warm. When ready to serve, sprinkle with parsley, if desired.
- With the rack in the highest position, preheat the oven to broil.
- Place the bread slices on a baking sheet. Top with the duck and cheese. Broil for 5 minutes or until the meat is heated through and the cheese has melted.
This soup freezes well without the cream.