Season the brisket with salt and pepper. Brown it in the oil for 5 minutes per side and place it fat-side up in a large slow cooker.
Cook the onions in the skillet for 7 minutes. Add the garlic and tomato paste and cook for 1 minute.
Stir in the crushed tomatoes, broth, Worcestershire, brown sugar and paprika. Scrape the skillet and pour the sauce around the brisket; add the bay leaves.
Cover and cook on low for 8 to 9 hours, until a fork slides into the thickest part easily but the meat is not falling apart. Discard the bay leaves.
Rest the brisket for 20 minutes and slice it thinly across the grain. Skim the sauce and spoon it over the slices, or chill the meat in sauce overnight and reheat gently the next day.