Unroll the boned leg of lamb and trim excess fat.

Reroll the lamb roast and secure with heavy string at 2-inch intervals.

Make 10 (1/2-inch-deep) slits in the surface of the roast.

Stuff garlic halves into the slits.

In a large zip-top plastic bag, combine orange juice, sherry (or orange juice), rosemary, salt, pepper, minced garlic, and bay leaf.

Add the lamb to the bag, seal, and marinate in the refrigerator for 8 to 24 hours.

Preheat oven to 450°F (232°C).

Remove lamb from the bag, reserving the marinade.

Discard the bay leaf.

Place the roast on a broiler pan.

Insert a meat thermometer into the thickest portion of the roast.

Sprinkle the roast with 1/2 teaspoon of salt.

Bake at 450°F for 20 minutes, basting once with 1/4 cup of the reserved marinade.

Reduce oven temperature to 400°F (200°C) (do not remove the lamb from the oven).

Add quartered oranges to the pan.

Bake for 1 hour or until the thermometer registers 145°F (medium-rare) to 160°F (medium).

Let the roast stand for 10 minutes before slicing.

Combine the pan drippings, the remaining marinade, and cornstarch in a small saucepan.

Bring to a boil and cook for 1 minute.

Stir in chopped parsley.

Serve the sauce with the sliced lamb.