Season & Sear the Beef: Pat the chuck roast completely dry with paper towels. Season aggressively on all sides with 1.5 teaspoons of salt and 1 teaspoon of cracked black pepper. Heat 2 tablespoons of olive oil in a heavy cast iron skillet over high heat until shimmering. Sear the roast for 4 to 5 minutes per side without moving until a deep, mahogany brown crust forms. Transfer the seared roast to a 6-quart or larger slow cooker.
Sauté Aromatics & Build Gravy Base: Reduce skillet heat to medium. Add the thick onion wedges and smashed garlic cloves, sautéing for 2 minutes until lightly browned. Stir in the tomato paste and cook for 1 minute until fragrant. Pour in 1/2 cup of the beef bone broth, scraping up all the golden browned bits (fond) stuck to the bottom of the skillet.
Layer into Slow Cooker: Pour the sautéed onion and garlic mixture over the seared roast in the slow cooker. Scatter the baby potatoes and chunky carrot pieces around and on top of the beef. Tuck in the fresh rosemary sprigs, thyme sprigs, and bay leaves.
Add Braising Liquid: Pour the remaining 1.5 cups of beef bone broth and the Worcestershire sauce around the edges of the slow cooker.
Slow Cook to Tenderness: Cover with the lid and cook on LOW for 8 to 9 hours (or on HIGH for 5 to 6 hours) until the beef is succulent, fork-tender, and effortlessly shreds with zero resistance.
Shred Beef & Finish Gravy: Carefully transfer the roast and vegetables to a warm serving platter using tongs, discarding the herb stems and bay leaves. Using two forks, shred the beef into generous hearty pieces. For a luxurious thick gravy, whisk the cornstarch slurry into the simmering cooking liquid in the slow cooker and cook on HIGH for 10 minutes until thick, glossy, and spoon-coating.
Assemble & Serve: Ladle the rich velvety gravy generously over the shredded beef, caramelized carrots, and tender potatoes. Shower with freshly chopped parsley and serve piping hot with crusty dinner rolls!