1.Pierce lamb 12 times with a sharp knife; press a slice of garlic and a piece of thyme into each cut. Reserve remaining garlic and thyme.
2.Preheat oven to 180°C (160°C fan-forced).
3.Heat half of the oil in a large flameproof baking dish; cook lamb until browned all over. Remove lamb from dish.
4.Heat remaining oil in same dish; cook onion, reserved garlic and thyme until soft but not coloured. Add tomatoes; cook, stirring, until softened. Add bay leaves, brandy and wine; bring to the boil.
5.Return lamb to dish; roast uncovered in oven for 30 minutes. Add potatoes to same dish; roast uncovered for further 1 hour 10 minutes or until lamb is cooked and potatoes are tender.
6.Spoon pan juices over the lamb occasionally during cooking. Remove lamb from dish. Cover with foil; stand 15 minutes.
7.Strain pan juices into medium saucepan; reserve tomato mixture and cover to keep warm. Skim fat from top of juices in saucepan. Add stock; bring to the boil. Boil, uncovered, 5 minutes or until reduced to 1½ cups (375ml).
8.Serve lamb with potatoes, reserved tomato mixture and sauce.