Pat the rump roast dry, season it with salt and pepper, and brown it in the oil for 4 minutes per side.
Put the onion, carrots and garlic in a 6-quart slow cooker and set the roast on top.
Combine the broth, Worcestershire, thyme and rosemary and pour it around the meat.
Cover and cook on low for 6 to 7 hours. Begin checking at 6 hours and remove the roast when a fork enters easily but the meat still holds together for slicing.
Rest the roast for 15 minutes. Stir the cornstarch slurry into the cooker and cook on high for 10 minutes, then slice the beef thinly across the grain and serve with the carrots and gravy.