Preheat the oven to 200°C.
Mix the garlic with the butter. Chop the rosemary leaves and mixed with garlic butter. Season with salt and pepper.
Make lots of incisions into the meat, then rub it with garlicy butter, massaging it into the holes with your fingers
Place the meat in a baking tray, then loosely cover it with foil.
Roast in the oven for 20 minutes, remove the foil and bake for about 45 minutes to your liking.
Remove the lamb from the oven - leave it in the baking sheet, cover it with the foil and rest it for 15 minutes.
Pour the cooking juice into a pan. Add the butter and the wine and bring to the boil. Take a couple of spoonful of the sauce into a small bowl and mix it with the cornflour into a smooth paste. Add this to the sauce and stir until you get a smooth, silky gravy.
Slice the lamb and serve with the sauce and the side dishes.