In a zip-top plastic bag, combine the lemon juice, rosemary, olive oil, and 1 tablespoon salt. Knead to mix, then add the lamb and knead the bag a little to coat the lamb. Refrigerate at least 3 hours, but overnight is best.
Preheat the broiler. Remove the lamb from the marinade and brush off any excess marinade. Make slits in leg of lamb, about 1/4-inch deep and about 1/2 inch apart. Stick a slice of garlic into each slit. Season with salt and pepper. Place the lamb under the broiler (it should be about 4 inches from the heat source) and broil for about 7 minutes on each side, until medium-rare.