Here's a fantastic way. Boil it. Yeah, boil it. Simmer the lamb in water with about a cup red wine, bay leaf, salt, a whole bulb of garlic, mirepoix, and rosemary until tender. This should take about two or three hours on the stove top. Remove the leg of lamb. Reduce liquid until half glace. To finish, put lamb in oven proof dish, place in oven at 375F and baste with combination of the reduced cooking liquid and a bit of clarified butter (or not). It is better if you cook the lamb early. You can leave it out without fear of dying from spoilage for at least a couple hours. Remove, serve with pan juices. The key is to get the glaze to stick to the lamb. You can also fortify the simmer liquid with a little demiglace, or a bit of broth base, beef stock. In the end it's the technique that counts.
Many years ago I did a slow roasted leg of lamb with white wine, not red, and about 5 - 6 HEADS of garlic, not cloves. I think it was based on something I saw in a Bon Appetit magazine, maybe around 1995. Of course there was other stuff in the roasting pan, and I remember cooking it for like 5 hours at 250F.
Maybe think of it as a braise and not a boil. A corned beefbrisket can be made tender by boiling for a while, though these days I roast mine. So maybe a simmer, rather than a vigorous, rolling boil might work for lamb. Or older, tougher mutton.