Preheat a water bath to 55°C. Lightly salt and pepper the lamb, then sprinkle with the garlic powder. Place the leg of lamb in a zip lock bag, position the rosemary sprigs on top, then seal. Place the bag in the water bath and cook for 4 hours, until heated through, or up to 24 hours until tenderized.
For the Roasted Vegetables
Preheat an oven to 200°C. Toss all the ingredients together with olive oil, then salt and pepper them. Place on a rimmed baking sheet, then cook, stirring once or twice, about 30 to 60 minutes, or until tender.
Take the sous vide bag out of the water bath and remove the cooked leg of lamb. Dry it off thoroughly, using paper towels or a clean dish cloth. Lightly salt the lamb, then quickly sear it for 1 to 2 minutes per side, just until browned. Remove from the heat. Cut the lamb into serving portions. Place the roasted root vegetables on a plate and top with a portion of lamb. Zest the orange on top, then serve.