You can cook your potatoes in the roasting pan around the roast and make a gravy using the juices. The only other thing you need is some fresh veggies. Watch this recipe being made in the Allrecipes Roast Lamb Video.
As a rule, roast a leg of lamb for 15 minutes per 500g, plus 10 minutes more. Allow it to rest for a good 15 minutes before serving to ensure the juices redistribute, making a perfect roast.
This simple way of roasting lamb is pretty much an institution in Australia.. its THE special family meal and there's nothing to compare. One important point though, unlike roasting a chicken,it's important to turn the leg over approx. half way through the cooking time (trust me on this, I have probably done this hundreds of times over 35 years). So, its a good idea to put the slits (stuffed with slivers of fresh garlic and sprigs of fresh rosemary) on both sides of the leg. Rub lightly with olive oil and throw in the oven. - 21 Jul 2008
Something else. very easy I made this leg of lamb for a party. I roasted my piece of lamb on 200 for one hour to brown, then added about a pint of water and covered the lamb, turned down the temp to 140 and let it slow cook for about 3 hours. 45 mins before it was done I added small new potatoes and 1 large onion sliced. I let it rest for about 20 mins before slicing. It was so tender you could cut it with a fork. I made a gravy out of the juices that were left. - 21 Jul 2008
Used different ingredients. I made this tonight for dinner and it was just wonderful! I rubbed olive oil on the lamb before adding the spices and used dried rosemary instead of fresh. I also cut slits in both the top and bottom of the leg of lamb for the garlic and used more garlic. For a 2.25kg leg of lamb, I roasted it uncovered at 180 for 2-1/2 hours. - 21 Jul 2008