When you buy ground lamb, ask that it be ground from the shoulder or shank. A mixture of these cuts is great, too. Ground lamb has enough body and flavor to stand on its own—no need to mix another ground meat with it—although we love seasonings such as garlic and rosemary.
Combine the lamb, mustard, rosemary, garlic, salt, and pepper in a large bowl. Using your hands, mix well. Form into 6 patties. Refrigerate until ready to grill.
Prepare a charcoal grill or gas grill. Lightly spray the grill rack with vegetable oil cooking spray. The coals should be hot.
Grill the burgers for about 7 minutes. Turn and grill for about 6 minutes longer for medium-well burgers.