Season the chuck roast with salt and pepper on all sides.
Layer half of the sliced onions in the bottom of the slow cooker, creating a thick bed.
Place the seasoned roast on top of the onions, then cover with remaining onion slices and scatter garlic slices throughout.
Combine beef broth, red wine, and Worcestershire sauce, then pour over the roast and onions.
Tuck thyme sprigs around the roast and dot the top with butter.
Cover and cook on low for 8 hours until the roast is tender and onions are caramelized and jammy.
Remove thyme sprigs, shred the roast if desired, and serve with the onions and sauce.