Cook the noodles as directed on the package. Drain the noodles and keep them warm.
Slice the beef into thin bite-sized strips. Mix the flour, salt and pepper in pie plate; toss beef strips in flour until coated.
Heat 1 tablespoon oil and 1 tablespoon butter in 12-inch heavy skillet. Brown half of the beef strips and the mushrooms over medium-high heat; remove to a clean bowl. Repeat with remaining oil, butter and beef; add to prior batch of beef. Add the onion, garlic and broth to the skillet; cook for 1 minute, scraping up brown bits from the bottom of the skillet.
Add the beef mixture back to skillet. Turn off the heat. Stir in the sour cream and simmer on low until heated through; do not boil.