Preheat the oven to 160°C / 140˚ Fan (300°F).

Chop the onion into 8 wedges (leaving the skin on) then make up the stock.

Rub the lamb all over with olive oil, then season generously with salt and pepper.

Make deep incisions about 1cm apart into the fat on the skin of the lamb. Chop the garlic into thin slices and insert the garlic slivers and rosemary sprigs into the incisions.

Place the chopped onion into the bottom of the roasting tin, then place the lamb on top of the onion wedges.

Pour the lamb stock and red wine into the tin. Pour around the lamb and not over it.

Cover the roasting tin tightly with foil.

Roast in the preheated oven for 4 - 4.5 hours, or until the lamb is tender and falling off the bone.

Remove the foil from the roasting tin & roast for an additional 30 minutes, or until the lamb is golden brown and crispy on the outside.

Remove the lamb from the oven and let it rest on a plate for 20-30 minutes before carving.

Follow the preparation instructions as above, using the slow cooker ceramic pot instead of a roasting tin.

Cook on the low heat setting for 8 hours or on high for 6 hours.

Transfer the remaining juices from the roasting pan to a large measuring jug. Using a spoon, carefully skim off any fat that has risen to the surface.

Sprinkle the flour into the roasting tray. Add a small amount of the skimmed fat, and stir with a wooden spoon until a roux forms.

Slowly pour the stock back into the roasting tin, and bring to a simmer, stirring until the gravy thickens.

Pour the gravy through a sieve to remove the onions and serve with the roasted lamb.