Preheat oven to 300 degrees.
Liberally dust roast with salt and pepper and pour cooking oil into Dutch Oven set to medium-high heat. Place roast in hot oil and sear on each side until browned.
Remove roast from Dutch oven and set aside on a plate.
Place chopped onions in Dutch oven (adding more oil if needed) and saute until tender and browning. Add minced garlic and cook for 1-2 minutes longer.
Stir in tomato paste and flour along with beef stock and Worcestershire sauce. Scrape any brown bits from the bottom of the pot.
Return roast to Dutch oven, cover with lid and place in the oven for 3 hours.
After three hours turn the roast and add rosemary sprig. Return to oven for an additional hour, or until tender.
Remove roast and set on a cutting board, tented with aluminum foil, and set Dutch Oven on the stovetop. Remove the rosemary sprig and skim fat, then bring juices to a boil and reduce the liquid to thicken into a sauce. Taste for salt and adjust as needed.
Slice the roast against the grain or shred as desired. Serve with thickened liquid from pot.