Season the roast with salt and pepper. Brown it in oil in a large skillet for 4 to 5 minutes on each broad side, then transfer it to a 6-quart slow cooker.
Sprinkle flour into the skillet and cook for 30 seconds. Slowly whisk in the stout and broth, then add tomato paste, mustard, Worcestershire and thyme.
Put the pearl onions and carrots around the roast. Pour over the stout mixture.
Cover and cook on low for 8 to 8 1/2 hours until the meat and carrots are tender. Transfer the roast and vegetables to a platter and skim the sauce.
Combine cornstarch and cold water, stir it into the sauce and cook on high for 10 minutes until glossy. Slice or pull the beef and serve with the vegetables and gravy.