How to Make It
Cook pasta according to package directions, omitting salt and fat.
While pasta cooks, combine broth, Worcestershire, vinegar, tomato paste, 1/2 teaspoon salt, and 1/8 teaspoon pepper in a medium bowl, stirring with a whisk.
Heat a Dutch oven coated with cooking spray over medium-high heat. Sprinkle beef with 1/2 teaspoon salt and 1/8 teaspoon pepper. Add beef to pan; cook 3 1/2 minutes on each side or until desired degree of doneness. Remove beef from pan.
Add onion and mushrooms to pan; sauté 3 minutes or until mushrooms are tender. Stir in flour; cook 1 minute, stirring constantly. Stir in broth mixture; cook 1 minute or until slightly thick, stirring constantly.
Cut beef into thin strips; return to pan. Stir in pasta, sour cream, and parsley; cook 1 minute or until thoroughly heated.