1.Preheat oven to 200°C (180°C fan-forced). Place 2 sprigs rosemary and 2 smashed garlic cloves in netting of each shoulder. Turn over, repeat.
2.Place lamb on a rack in a large roasting pan. Bake 20 minutes. Reduce oven to 160°C (140°C fan-forced), bake 1 hour 30 minutes. Turn over; bake 20 minutes. Cover with foil: rest for 15 minutes. Slice enough lamb for one meal; chill remaining lamb.
3.Preheat oven to 200°C (180°C fan-forced). Line 2 large baking trays with baking paper. Place vegetables, except garlic, on prepared trays. Spray with oil, toss to coat. Bake 1 hour. Turn vegetables and swap trays halfway through cooking. Add garlic heads to tray.
4.Reheat sliced lamb in oven. Make gravy as packet directs. Place lamb on serving plates with potato and a third of the other vegies and garlic; chill remaining vegies and garlic. Serve with green beans and gravy.
We used lamb shoulder, boned, rolled and secured in netting. Slice enough meat for one meal then cover with foil; reheat just before serving.Note