Spray slow-cooker bowl with nonstick cooking spray. Season roast with salt, thyme and black pepper. Place in bottom of slow cooker.
Scatter onion, carrots, celery and garlic over roast. Pour stock and balsamic vinegar over top. Cover and cook on LOW for 10 hours.
Remove roast and allow to rest 15 minutes before slicing. Skim fat from cooking liquid. Mash vegetables in cooking liquid to slightly thicken sauce.
Serve sliced roast with sauce and, if desired, polenta and sautéed peppers.
