Pat the rump roast dry and trim excess fat, leaving a thin layer for flavor.
Mix salt, pepper, garlic powder, onion powder, paprika, and thyme.
Rub the seasoning mixture over the entire roast.
Let the roast rest for 30–45 minutes.
Heat oil and sear the roast on all sides until browned.
Place onions, carrots, and celery in the slow cooker or Dutch oven.
Add beef stock, Worcestershire sauce, tomato paste, and mushroom powder.
Place the roast on top of the vegetables.
Cook on low for 8–10 hours (or in a Dutch oven at 300°F / 150°C until tender).
Remove the roast and let it rest for 15–20 minutes before slicing.
Collect drippings and prepare gravy by whisking in flour and stock until thickened.
Slice against the grain and serve with gravy.