Mix the garlic, rosemary and rapeseed oil in a bowl. Put the lamb in a shallow dish (or use a turkey bag if you have one). Rub the lamb with the oil & garlic marinade. Leave to to marinade for 3-4 hours, overnight is best.
Preheat the oven to 190°C (375°F/gas mark 5).
Using a japanese mandolin, carefully slice the sweet potatoes and rooster potatoes. Layer the base of a lined roasting tray with the slices. Place the garlic gloves, thyme and rosemary on top followed the stock.
Bring the lamb back up to room temperature. Heat a large non-stick fryign pan until smoking hot. Gently place the lamb skin side down. Leave for 2-3 minutes until well coloured and then turn over. Transfer to the prepare roasting tray. Roast in the oven for 45-50 minutes. If you prefer your lamb more well done, cook it for an extra 15–20 minutes.
Remove the lamb from the oven and rest in a warm place for 10 minutes. If you don’t like your lamb too pink, cover it with foil and it will continue to cook. Carve into slices and serve.