Season the roast with the salt and pepper and brown it in the oil in a hot skillet for 5 minutes per broad side.
Spread the onion, carrots, parsnips and potatoes across the bottom of a 6-quart slow cooker and set the roast on top.
Whisk the tomato paste, broth, Worcestershire, rosemary and thyme and pour the mixture around the beef.
Cover and cook on low until the meat is fork-tender and the root vegetables are soft but intact, 8 to 8 1/2 hours.
Move the roast and vegetables to a platter, tent loosely with foil and skim the fat from the liquid.
Whisk the cornstarch with the cold water, stir it into the cooker and cook on high until glossy and thickened, about 10 minutes.
Break the roast into serving pieces and spoon the gravy over the meat and vegetables.