A butterflied leg of lamb, which cooks quickly and evenly on the grill, has been split down the center, then unfolded and spread open to resemble a butterfly.
In a medium bowl, whisk together oil, lemon juice, vinegar, rosemary, garlic, 2 teaspoons salt, and 1/2 teaspoon pepper. Place lamb in a large shallow dish with marinade, and turn to coat. Cover; marinate 1 hour at room temperature or up to overnight in refrigerator, turning occasionally. (Bring to room temperature before grilling.)
Heat grill to medium-high. Thread several metal skewers through lamb in both directions to hold meat together during cooking.
Grill lamb until an instant-read thermometer inserted into the thickest part of the meat registers 140 degrees for medium rare, 10 to 15 minutes per side. Let the meat rest, covered, about 10 minutes. Remove the skewers. Slice the meat thinly, against the grain where possible.
Skewer the meat in both directions to keep it together. That way, it will be easier to manage once it's on the grill.