Pat the chuck dry and season it all over with the salt and pepper. Heat the oil in a large skillet over medium-high heat and brown the roast for 4 to 5 minutes per broad side.
Put the onion, carrots, potatoes and garlic in a 6-quart slow cooker. Set the browned roast on top.
Cook the tomato paste in the skillet for 1 minute. Add 1 1/2 cups broth, Worcestershire and thyme, scrape up the browned bits, and pour the liquid around the roast. Add the bay leaves.
Cover and cook on low for 8 to 9 hours, until the beef pulls apart easily and the potatoes are tender. Move the meat and vegetables to a platter and discard the bay leaves.
Whisk the flour with the remaining 1/2 cup cool broth. Stir it into the pot liquid, cover and cook on high for 10 to 15 minutes until thickened. Slice or pull the roast and serve with the gravy.