Cook pasta according to package directions. Meanwhile, sprinkle salt and 1/4 teaspoon pepper evenly over steak in medium bowl; toss to coat. Set aside. Combine sour cream, flour and tomato paste in small bowl. Stir in water, bouillon and 1/8 teaspoon pepper; set aside.
Spray large skillet with cooking spray; heat over medium-high heat 30 seconds. Add steak; cook 3 to 4 minutes or until no longer pink in centers, stirring frequently. Remove steak from skillet, reserving any juices in skillet. Cover to keep warm.
Add mushrooms and onion to same skillet; cook 3 to 4 minutes or until onion is crisp-tender. Stir in sour cream mixture; cook 2 minutes or until thickened and bubbly, stirring constantly. Return steak to skillet. Cook until heated through, stirring occasionally. Drain pasta; place on large serving platter. Top with steak mixture; sprinkle with parsley.