Fennel cooked sous vide becomes very tender and retains the majority of its flavor. The length of time you cook it for depends on how tender you want it. At 30 minutes it's just beginning to tenderize, after 60 minutes it retains a little bite, and after 90 minutes it is completely tender. Cooking the fennel with a large dose of olive oil confits the fennel, resulting in a tender, juicy fennel. The excess olive oil can be used on other dishes or in vinaigrettes and will last in the refrigerator for several days.
Preheat the water bath to 183°F (83.9°C).
Trim the fennel bulbs, cut them in half, then place them in a sous vide bag in a single layer. Add the garlic and saffron to the bag. Zest the orange into the sous vide bag then salt and pepper the fennel. Add the olive oil and seal. Cook the fennel for 60 minutes. Once tender, remove the fennel from the sous vide bag then serve.