Heat the avocado oil in a large cast iron skillet over medium-high heat. Season both sides of the beef chuck roast with salt and black pepper, then sear for 4 to 5 minutes per side until a dark brown crust forms. Transfer the seared roast to a 6 to 7-quart slow cooker.
Add the sliced garlic, onion chunks, carrots, celery, potatoes, rosemary sprigs, thyme sprigs, and bay leaves to the slow cooker with the roast. Pour in the beef broth and red wine.
Cover with the lid and cook on low for 8 to 9 hours or on high for 5 to 6 hours.
Remove and discard the rosemary sprigs, thyme sprigs, and bay leaves. Transfer the roast to a board or platter and slice or shred the meat.
Optional: To thicken the cooking liquid into a gravy, whisk together the cornstarch (or arrowroot powder) and water in a small bowl until smooth. Stir the slurry into the slow cooker until the liquid thickens.
Serve the shredded or sliced beef and vegetables on a platter with the cooking juices or thickened gravy.