Heat 1 tablespoon oil in large skillet over medium-high heat. Season meat with 1/2 teaspoon salt and 1/8 teaspoon pepper. Working in batches, add beef to skillet; stir-fry 2 minutes per side adding another tablespoon oil as needed. Remove beef strips to a plate and keep warm.
Add 1 tablespoon oil to the skillet; heat. Add onions; cook over high heat, stirring frequently, for 5 minutes. Add mushrooms; cook for 6 to 7 minutes or until heated through.
Meanwhile, whisk together 1 cup broth, the sour cream, paprika and mustard in small bowl until smooth.
Add remaining 2 cups beef broth, the tomato paste, thyme and the remaining salt and pepper to the onion mixture in skillet. Bring to a simmer, stirring to combine. Stir in the sour cream mixture and the reserved beef strips. Gently heat through, stirring occasionally, about 3 minutes; do not let come to a simmer.
Place hot cooked noodles in large bowl. Pour the beef mixture over the top; gently stir to combine. Tent with aluminum foil; let stand 5 minutes before serving to absorb sauce.