Dry rub: add the garlic powder, onion powder, mustard powder, paprika, salt and black pepper to a small bowl and stir until consistently combined.
Beef: remove any packaging from the beef then use a folded piece of paper towel to pat it dry. Also remove any butchers string. Place it in the centre of a clean, dry chopping board.
Massage: sprinkle the dry rub over each side of the beef chuck, massaging it in as you go. Ensure the whole cut is covered in the rub.
Grease: lightly grease the slow cooker insert with olive oil spray.
Sauce: add the beef stock, barbecue sauce, tomato sauce, crushed tomatoes and Worcestershire sauce to the slow cooker. Use a wooden spoon to stir and gently combine.
Beef and sauce: now place the seasoned beef into the slow cooker and put the lid on. Set the timer and temperature to 4 hours on high or 7 hours on low.
Slurry: combine the cornflour with the water in a small jug and stir till consistently combined.
Remove beef: once the initial cook time is up, remove the lid from the slow cooker. Then transfer the cooked beef to a chopping board. Use 2 forks to shred the beef.
Thicken sauce: pour the slurry into the sauce and stir to combine.
Add beef to sauce: now add the shredded beef back to the slow cooker and put the lid on. Set the timer to 1 hour on high or 1 ½ hours on low.
Separate: once the final cook time is up, remove the lid of the slow cooker. Use a sieve or colander with a jug under it and pour the beef into the sieve using the jug under to collect the sauce.
To serve: serve the beef with creamy mashed potatoes and drizzle desired amount of sauce over the top.