1. Trim and discard fat from beef. Cut the meat across the grain into 1/8-inch-thick slices.
2. Place a 12- to 13-inch nonstick frying pan over high heat. When pan is hot, add beef and stir-fry until meat is lightly browned, 2 to 3 minutes. Pour into a bowl.
3. Add mushrooms to pan and stir often until browned, 3 to 4 minutes. Add to beef.
4. Add onion to pan and stir until lightly browned, about 2 minutes. Reduce heat to medium and stir paprika into onion.
5. Meanwhile, cook noodles in about 3 quarts boiling water until barely tender to bite, 4 to 5 minutes. Drain and place on a large platter; keep warm.
6. Mix broth and cornstarch; add to onion mixture and stir just until mixture boils, about 2 minutes.
7. Return beef and mushrooms to pan and stir until hot, about 1 1/2 minutes. Add sour cream and stir just until hot, about 1 minute.
8. Spoon meat mixture over hot noodles. Sprinkle with chopped parsley and season to taste with salt and pepper.