1. Begin with the fattier side facing up. Using a sharp, thin knife, remove some, but not all, of the surface fat to prevent flare-ups.
2. Turn the leg over. Cut away any large clumps of fat.
3. Slide the tip of your knife under the large areas of sinew and carefully work the knife between the sinew and meat.
4. For slightly thicker sections of the leg, make an angled cut at the thickness you want and spread the meat open like a book.
5. If one section of the leg is considerably thicker than the rest, cut it off and grill it separately.
6. In the end, you should have a leg of lamb with a relatively even thickness and 1 or 2 small sections to grill separately.