Season the roast with salt and pepper. Heat the oil in a skillet and brown the roast for 4 to 5 minutes per side; transfer it to a 6-quart slow cooker.
Melt the butter in the skillet. Add the onions and sugar and cook over medium heat for 18 to 22 minutes, stirring often, until deep golden. Stir in the garlic for 30 seconds.
Sprinkle flour over the onions and cook for 1 minute. Stir in the broth, wine and Worcestershire, scraping the pan clean.
Pour the onions and liquid around the roast. Add the thyme and bay leaves, cover, and cook on low for 8 hours until the beef is fork-tender.
Discard the bay leaves, skim the sauce and rest the beef for 10 minutes. Slice or pull it and serve with plenty of onions and sauce.