Cut the base off of the fennel bulbs, and cut a cone shape into the base to remove the core. You can see the core because it is whiter than the surrounding green. This is optional, but helps the fennel cook faster. Slice the fennel vertically (upright) into 5mm/1/4 inch thick slices.
Melt the butter in a large frying pan over medium heat. Add the fennel, and fry for about 5 minutes. Stir in the cream and creme fraiche until well blended. Transfer to a shallow baking dish. Sprinkle Parmesan cheese over the top.
Bake for 30 minutes in the preheated oven, or until the top is golden brown and the fennel is tender enough to pierce with a fork.