Preheat the oven to 220°C.
Insert sliced garlic into small holes cut through the lamb skin.
Roast the lamb for 20 minutes, turn it over, and roast for 25 more minutes until brown.
Boil onions for 10 minutes, add potatoes for 4 minutes, then drain.
Place vegetables and 570ml stock in the tin with the lamb on top and cover with foil.
Reduce oven to 160°C and cook for 4 hours until tender.
Brown the potatoes and onions at 220°C for 25 minutes while resting the lamb.
Make gravy by whisking fat, flour, 450ml stock, wine, and seasonings over heat until thick.
Carve the lamb and serve with the gravy, potatoes, and onions.