This leg of lamb, redolent of garlic and oregano, can be marinated overnight and then given a quick turn on the grill while your dinner guests mingle.
Cut ten 1/2-inch-long, 1/2-inch-deep slits all over lamb. Insert a garlic clove half into each slit. Season lamb all over with pepper and 3 tablespoons salt.
Combine oil, lemon juice, oregano, and minced garlic in a large, resealable plastic bag. Add lamb, and turn to coat. Seal bag, and place in a large bowl. Refrigerate overnight (or up to 2 days).
Let lamb stand at room temperature for 1 hour. Season with salt and pepper. Preheat grill to medium-high. (If you are using a charcoal grill, coals are ready when you can hold your hand 5 inches above grill for just 3 to 4 seconds.)
Grill lamb to desired doneness. (For medium-rare, grill until a meat thermometer inserted into thickest portion registers 130 degrees, about 15 minutes per side.) Transfer to a cutting board, and let rest, uncovered, for 15 minutes.
Meanwhile, place lemon slices on grill. Cook over medium-high heat until caramelized and soft, about 10 minutes per side. Slice lamb. Garnish with oregano sprigs and lemon slices.