Sauté the fennel, carrots, onion and potato in the butter. Add some fresh thyme and 1 bay leaf. Cover and simmer on medium low for 45 minutes to an hour. Add the vegetable broth and bring it to a boil. Add salt and pepper to taste and simmer 8-10 mins. Let the soup cool, then remove the herbs and purée it in a blender. Mix in a pinch of maple syrup and creme fraiche to taste when serving and sprinkle with chestnuts.