Season the rump roast with sea salt and ground black pepper and let sit until it comes to room temperature (about 15 minutes).
In the meantime, add the chopped yellow onions, carrots, baby potatoes, garlic, oregano, and bay leaves to the bowl of a slow cooker. Set aside.
Heat the avocado oil in a heavy-bottomed pan over medium-high heat.
Place the seasoned rump roast in the pan and let sear for 2 minutes before flipping and searing for 2 minutes on the other side.
Transfer the seared rump roast to the slow cooker; pour in the beef stock. The liquid should cover about 3/4 of the rump roast. If needed, add more broth.
Close the slow cooker and cook on low heat for 8 hours, flipping the roast halfway through.
When cooking time is up, remove the beef from the slow cooker and transfer it to a cutting board. Shred the rump roast using two forks or meat shredders - it should be fall-apart tender. If it's not pulling easily, place it back in the slow cooker and continue to cook for 30 more minutes and up to one hour.