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 and season with salt and 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 and simmer 1 minute or until sauce is thickened, stirring frequently. Return Beef and heat through. Serve over noodles with sour cream.