01 - Pat the beef roast dry with paper towels and season all sides evenly with salt and pepper. Heat a large skillet over medium-high heat and sear the roast on all sides until deep brown, approximately 3 to 4 minutes per side. Transfer to the slow cooker.
02 - In the same skillet, add sliced onions and sauté for 3 to 4 minutes until softened and browned at the edges. Add minced garlic and cook for 1 additional minute. Transfer the onion mixture to the slow cooker.
03 - Distribute the carrots and celery pieces around the beef roast in the slow cooker.
04 - In a mixing bowl, whisk together beef broth, red wine if using, tomato paste, and Worcestershire sauce until well combined. Pour the mixture evenly over the roast and vegetables.
05 - Place thyme, rosemary, and bay leaves into the slow cooker, distributing them throughout the ingredients.
06 - Cover the slow cooker and cook on LOW setting for 8 hours until the beef yields easily to a fork and all vegetables are tender.
07 - Transfer the beef and vegetables to a serving platter using tongs. Discard the herb sprigs and bay leaves from the cooking liquid.
08 - If desired, mix cornstarch with cold water to create a slurry. Stir the slurry into the slow cooker liquid, cover, and cook on HIGH for 10 to 15 minutes until the sauce reaches the desired thickness.
09 - Slice or shred the pot roast and arrange on the serving platter. Spoon the caramelized onions and rich gravy over the meat and vegetables. Serve immediately.