Cook noodles according to package directions, omitting salt.
Meanwhile, spray 10-inch skillet with cooking spray; heat over medium-high heat. Add beef; cook 5 minutes or until browned, stirring and turning strips. Add onion and mushrooms; cook 3 to 4 minutes or until tender. Reduce heat to medium; stir in gravy and ketchup. Cook until bubbly. Remove from heat; gently stir in sour cream.
Divide noodles between 2 plates. Top each evenly with beef mixture.