01 - Heat a large skillet over medium-high heat. Add sliced beef sirloin and sear on all sides until well-browned. Transfer beef to a plate and set aside.
02 - In the same skillet, reduce heat to medium. Add sliced mushrooms and diced onion, sautéing until vegetables are softened and onions are translucent.
03 - Sprinkle flour evenly over the mushrooms and onions. Stir well to incorporate, cooking for 1 minute to eliminate any raw flour taste.
04 - Gradually pour in beef broth, stirring continuously to prevent lumps. Bring the mixture to a gentle simmer and cook until slightly thickened, about 5 minutes.
05 - Return the browned beef and any accumulated juices to the skillet. Reduce heat to low and fold in sour cream, mixing thoroughly until sauce is creamy and beef is heated through. Do not boil after adding sour cream.
06 - Arrange cooked egg noodles on serving plates and spoon the beef and mushroom sauce over top. Serve immediately.