Heat the oven to 325°F and season the roast all over with the salt and pepper.
Heat the oil in a Dutch oven over medium-high heat and sear the beef until a dark brown crust forms, 5 to 6 minutes per broad side; transfer it to a plate.
Cook the onion in the pot until browned at the edges, 5 minutes, then stir in the tomato paste and flour for 1 minute.
Whisk in the broth, scraping every browned bit from the bottom, and stir in the Worcestershire, thyme and bay leaf.
Return the roast and its juices to the pot, cover and braise in the oven for 2 hours.
Arrange the carrots around the beef, cover again and cook until the meat pulls apart with a fork and the carrots are tender, 60 to 90 minutes more.
Transfer the meat and carrots to a platter, discard the bay leaf and skim excess fat from the liquid.
Simmer the gravy on the stovetop until it coats a spoon, 5 to 10 minutes, then slice or pull the roast and spoon gravy over it.