Add sliced onion to the bottom of the crockpot.
Place the rump roast on top of the onions.
Sprinkle salt, pepper, smoked paprika, garlic powder, and onion powder evenly over the roast.
Pour in beef broth, Worcestershire sauce, and tamari (soy sauce). Add minced garlic and butter (if using).
Cover and cook on LOW for 8 hours or HIGH for 5–6 hours, until the roast is fork-tender.
Remove the roast and shred with two forks.
Return the shredded beef to the crockpot and stir into the juices. Let sit for 10–15 minutes.
Serve warm on sandwiches, in quesadillas, or however your family prefers.