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