Season the roast with the salt and pepper and heat the oil in a large skillet over medium-high heat.
Sear the beef until a dark crust forms, 5 to 6 minutes per broad side, then transfer it to a 6-quart slow cooker.
Arrange the onion, potatoes and carrots around the roast.
Whisk the flour with 1/2 cup broth until smooth, then whisk in the remaining broth, Worcestershire, onion powder and thyme.
Pour the mixture around the beef, cover and cook on low until the roast slumps under tongs and the vegetables are tender, 8 to 8 1/2 hours.
Move the beef and vegetables to a platter, skim the cooking liquid and switch the cooker to high.
Cook the gravy uncovered until it coats a spoon, 10 to 15 minutes, then pull the roast into serving pieces and spoon the gravy over it.