Spray crockpot with non-stick spray.
Sprinkle Montreal Steak Seasoning over both sides of roast, rubbing it into the meat with your fingers.
Place roast in bottom of slow cooker.
Sprinkle rosemary over roast.
Pour the jar of beef gravy over roast.
Top with chopped onion.
Cover and cook on low 7-8 hours or on high for 3-4 hours.