Saute the fennel, onion, carrots, potato, thyme and bay leaf in butter until soft but not brown. Add the broth, bring to a boil, then simmer until the potatoes are falling apart. Remove the herbs, puree in a blender until smooth, then strain out the solids.
Mix the sour cream and maple syrup.
Plate the soup, garnish with the sour cream mixture, and serve.