Lightly grease a 4- to 6-quart slow cooker with nonstick spray.
Spread sliced mushrooms evenly across the bottom.
Place beef pieces in a single even layer over the mushrooms.
Mix condensed mushroom soup and onion soup mix in a bowl until smooth.
Spread the mixture evenly over the beef, fully coating all pieces.
Cover and cook on LOW for 8 hours or HIGH for 5 hours until beef is fork-tender.
Stir gently to combine beef, mushrooms, and gravy.
Let sit on WARM for 15 minutes to thicken slightly, then serve.
