The most important thing to remember about cooking a lamb roast is not OVER-COOK it. Lamb has such wonderful flavor on its own, and is so naturally tender, that it is bound to turn out well, as long as it is still a little pink inside.
There is some debate over which method yields the best results - slow cooking at low heat the entire time, or searing first on high heat and then slow cooking. James Beard in his American Cookery prefers the slow-cook-low-heat method (he rubs the roast with salt and pepper and cooks it at 325°F the whole time.)
You will also get great results with the searing method, starting at high heat and then dropping the temp which is the method described in this recipe.
For this roast, pull it out at 130°F if you like it medium rare. As it rests the internal temperature will continue to rise a few points as the meat continues to cook.(I like lamb on the rare side of medium rare, and this roast was perfectly done for me). An accurate meat thermometer is essential.
The marinade used works quite well, however; a simpler marinade of rosemary, olive oil, garlic, pepper, and lemon juice would do the trick quite nicely too. You can also make tiny slices into the surface of the roast and insert slivers of garlic and herbs if you like.
NOTE: Preparation time DOES NOT include the marinating time overnight in the refrigerator!