In a small bowl, mix 1 ½ leek with scallions and prosciutto and set aside. Carefully insert finger under pheasant skin at neck, creating a space between breast meat and skin.
Repeat at other end, below ribs. With sharp knife, make 2 or 3 slits in skin and flesh of both legs on all sides. Stuff as much of leek mixture as possible under skin and into slits.
Combine any excess mixture and stuff inside pheasant cavity. Pour champagne or wine into cavity and truss bird with butcher's twine or stuffing skewers.
Place pheasant in large roasting pan and pour remaining champagne or wine over bird. Rub skin with olive oil and sprinkle with salt, pepper and parsley. Cover and roast at 250 degrees for about 1 hour. Remove cover and baste; bake, uncovered for another 30 minutes or until meat thermometer inserted in drumstick flesh registers 160 degrees.
Serve pan drippings and keep mixture ladled over meat.