Heat oven to 450 degrees. Scrub and halve the potatoes; set aside. Clean the garlic. Cut 4 to 5 cloves in half and roughly chop the rest.
With a sharp knife, trim all fat from the leg of lamb. Wash with lemon juice and place in large roasting pan.
Make scores, about 1/4″ deep, over entire top of lamb.
Insert halved garlic cloves with a sharp knife at various intervals into leg. Spread chopped garlic over top of lamb and season with oregano, rosemary and pepper.
Arrange halved potatoes around lamb, cut side up.
Roast at 450 degrees for about 20 minutes to sear. Lower heat to 325 degrees and roast for an additional 1 1/4 hours or until meat thermometer registers 160 degrees for medium. Turn potatoes halfway through roasting time.