Rub beef all over with oil, place in a 10- by 14-inch roasting pan, and bake in a 450* oven until well browned (about 45 minutes), turning often. Then reduce oven temperature to 400 degrees F. Add broth and port to pan, cover tightly, and bake for 1 1/2 hours.
Mix sugar and onions into pan juices; cover tightly and bake for 1 more hour. Stir in cranberries; cover tightly and bake until beef is tender when pierced (about 30 more minutes).
Transfer beef to a platter; spoon noodles alongside. With a slotted spoon, ladle onions and cranberries onto noodles; keep warm. Skim and discard fat from pan juices; then bring juices to a boil over high heat. Stir in cornstarch mixture; cook, stirring, until sauce is bubbly and thickened. Season to taste with salt and pepper, then pour into a small bowl.
To serve, thinly slice beef across the grain. Accompany beef and noodles with sauce.