If time permits, season roast with all purpose seasoning and let rest for one one hour before placing into slow cooker.
Mix bouillon and hot water together and stir until disolved. Place roast beef into the pot of your slow cooker, add beef broth and seasoning. Cover with sliced onions. Cook roast in slow cooker for 3-4 hours on High setting or 6-8 hours on Low.
After cooking, remove roast from slow cooker and to a platter. Pull apart roast with two forks. Return to the slow cooker if not serving right away, keeping it on warm setting.
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Notes
Round roast and rump roast can be also be used with less predictable results. Use well-marbled roasts and be prepared to chop the cooked meat if it doesn't easily shred.