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.
I once got a bunch of lamb necks cheap. Simmered till tender, then bones into a crock pot along with other lamb bones I'd saved, were they stayed for 36 hours, replacing lost liquid every day. Reduced that, and it was the most amazing stock I every made, intense lamb flavor, God! In went the meat with just salt and pepper. It will work great with lamb leg, and isn't far from what Kuan described.
I former member once suggested punchig narrow slits in and stuffing with galic and anchovie, works great. Basil and fennel seed together great. Braised with spanish espelet peppers. Throw juniper berries in there. Just mint.
I do mine Greek style. I usually take off the shank and save that for a slow braise. At this point you can butterfly it and remove the gland then roll and tie, or just roast it whole. (it might be a little gamey with the gland in) Salt and pepper then cut slits and insert garlic slices and rosemary sprigs. Place in a dry pan in a hot oven 425F for 15 minutes then reduce the heat to 325 - 350 and roast uncovered for 2 - 2-1/2 hours and let rest. 45 minutes before the end of cooking you can add potato wedges and squeeze lemon over it, or you can add tomato juice and orzo, but still squeeze lemon over. Rest and enjoy -