Heat up the oil in a large pot, add the beef, season it with half of the salt and pepper, and sear it on all sides until brown.
Remove it from the pot and transfer it to the slow cooker.
In the same pot, add the peeled and chopped onion and garlic with the remaining salt and pepper, and fry them until golden.
Pour in the beef stock, add the tomato paste, Worcestershire sauce and bay leaves, give it a stir, then pour everything over the beef.
Cook it on high for 5-6 hours or on low for 9-10 hours.
Once the beef is cooked, mix the cornflour with 2 tablespoons of water to make a slurry, then add it to the liquid in which the beef cooked.
Remove the beef from the slow cooker, wrap it in foil, and allow it to rest while the gravy is thickening.
To speed up the process, you can also transfer the gravy to a pan set over a medium heat on the cooker, this will thicken a lot faster, 2-3 minutes max, in the slow cooker it can take up to 30 minutes.
Slice or shred the beef, and serve it with the onion gravy.