Cook noodles according to package directions; keep warm.
Meanwhile stack Beef steaks; cut lengthwise in half and then crosswise into 1"-wide strips.
In large nonstick skillet, heat 2 teaspoons oil over medium-high heat until hot. Add Beef and garlic (1/2 at a time) and stir-fry 1 minute or until outside surface of Beef is no longer pink. (Do not overcook.) Remove from skillet; season with 1/4 teaspoon salt and 1/4 teaspoon pepper.
In same skillet, cook mushrooms in remaining 2 teaspoons oil 2 minutes or until tender, stirring occasionally; remove from heat. Stir in gravy mix and 1 cup cold water; bring to a boil. Reduce heat to low; simmer 1 minute or until sauce is thickened, stirring frequently. Return Beef; heat through. Serve over noodles with sour cream.
