Place beef and flour in a 1-gallon recloseable food storage bag; salt and pepper to taste. Seal bag; toss to coat beef. Remove beef from flour; discard excess flour.
In large skillet over medium-high heat, heat oil. Add beef and brown on all sides (about 3-5 minutes). Transfer to a slow cooker.
Add beef gravy, mushrooms and onions to slow cooker; stir to combine. Cook on LOW 6-8 hours or until beef is tender.
Serve over mashed potatoes or cooked egg noodles.