Adjust oven rack to lowest position; preheat oven to 400°. Remove neck, giblets and liver from turkey cavity; discard liver. In large roasting pan, place 4 oranges, 4 onions, neck and giblets; place turkey, breast side up, on top. Stuff turkey cavity with rosemary sprigs and remaining 1 each orange and onion.
In medium bowl, stir butter, orange zest, 1 teaspoon salt and remaining 1/2 tablespoon chopped rosemary. Work your fingers between the skin and breast to loosen skin; evenly distribute butter mixture under skin. If not already secured, tie legs with kitchen string and tuck wing tips under turkey to hold in place. Roast turkey 25 minutes or until outside of turkey is browned.
In small saucepan, whisk cranberry sauce, syrup, orange juice and mustard. Heat to a simmer over medium heat; simmer 5 minutes, whisking occasionally. Makes about 1 cup.
Remove turkey from oven; brush with about 3 tablespoons sauce mixture. Reduce oven temperature to 325°; tent turkey loosely with aluminum foil and roast 2 1/2 hours or until juices run clear and internal temperature reaches 160° in thickest part of thigh, brushing with glaze every 30 minutes. Transfer turkey to cutting board; loosely tent with foil and let stand 30 minutes. (Internal temperature will rise to 165° upon standing.)
Remove oranges and onions in roasting pan; with spoon, skim excess fat from drippings. Place roasting pan on stovetop over medium heat. Add stock and heat to a simmer; simmer 10 minutes, scraping browned bits from bottom of pan with wooden spoon. In small bowl, whisk corn starch and 1/4 cup cold water; whisk into pan and simmer 1 minute or until slightly thickened, whisking constantly. Remove from heat; whisk in pepper and remaining 1 teaspoon salt. Remove neck and giblets; strain gravy through fine-mesh strainer, if desired. Makes about 3 cups.