Place your leg of lamb on a chopping board, keeping the string attached. Using a sharp knife, make incisions around the entire leg of lamb, and push in slices of garlic or rosemary intermittently. Ensure that the garlic and rosemary is evenly distributed across the entire lamb leg, and sprinkle with salt and pepper to taste. 

Pour the olive oil over the lamb and rub in to ensure it is covered. Heat a fry pan over a medium heat and add the lamb leg to the fry pan. Brown to sear on all sides of the lamb leg, only allowing to slightly brown for a minute or so until you move to another side. Remove from heat when it is completely browned.

Juice the lemon and place the lemon skins in the bottom of a slow cooker. Add the chopped onion, and then place the lamb leg on top. Pour the lemon juice over the lamb, and add the water into the bottom of the slow cooker. 

Cook the lamb in the slow cooker for about 8 hours on low, or 4 hours on high. The lamb should be cooked through and slightly falling apart. 

Preheat the oven to 200C (400F). Place the lamb leg in a baking dish and cover with foil. Cook in the oven for about 20 minutes, or until the top of the lamb is golden. 

Meanwhile, pour the juice from the slow cooker into a fry pan, removing the lemon skins. Simmer on a low heat until it has reduced slightly. 

To serve, remove the lamb from the oven and cut off the string. Slice into pieces and place on a serving platter, pouring over the juice from the fry pan evenly. Serve with potatoes, roasted vegetables or salad.