Cut the bottom and top off the fennel and cut into quarters. Also add some salt if the stock was unsalted. Vacuum seal with frozen stock. (Freezing the stock allows you to use a FoodSaver-type vacuum sealer. If you have a chamber vacuum sealer, you don’t need to freeze the stock first.)
Drain the liquid from the bag into a bowl.
Pat the fennel dry with paper towels.
Heat some clarified butter or olive oil in a frying pan and quickly brown the fennel over high heat. Add the juices from the bag. Cook until the juices have reduced to a syrupy sauce, turning and basting the fennel in the sauce.
Serve with the sauce spooned over the fennel.