1.Preheat the oven to 220°C (200°C fan-forced).
2.Heat half of the oil in a large flameproof baking dish; cook lamb until browned all over. Sprinkle lamb with salt and pepper. Place rosemary sprigs under lamb, add garlic and onion to pan. Roast, in oven, for 20 minutes or until cooked as desired. Remove lamb from dish; cover, stand for 10 minutes.
3.Meanwhile, place parsnip, carrot and potato in a separate shallow oven tray, toss with remaining oil and scatter with butter pieces. Roast for about 30 minutes or until tender and golden brown. (Once the lamb is removed from oven, increase the temperature to its hightest setting for the last 10 minutes of cooking time.)
4.Add flour to the lamb baking dish; cook, stirring, over heat until mixture bubbles and is well browned. Gradually stir in stock; bring to the boil, stirring, until gravy thickens. Strain gravy into a small jug.
5.Serve sliced lamb with roasted vegetables, gravy and steamed broccolini, if desired.