Season & Sear: Pat the beef chuck roast completely dry with paper towels. Generously rub on all sides with kosher salt, black pepper, garlic powder, and onion powder. Heat olive oil in a heavy cast-iron skillet or Dutch oven over high heat. Sear the roast undisturbed for 4-5 minutes per side until a deep, dark golden-brown crust develops. Transfer seared roast into slow cooker.

Deglaze the Skillet: Reduce skillet heat to medium. Add tomato paste and minced garlic, stirring for 60 seconds until fragrant. Pour in 1/2 cup of beef bone broth, scraping up all browned savory bits (fond) from bottom of skillet. Pour entire skillet juices into slow cooker over roast.

Assemble Vegetables & Liquids: Scatter potato halves, carrot chunks, and onion wedges around and over beef roast. Whisk remaining beef bone broth with Worcestershire sauce, balsamic vinegar, and Dijon mustard; pour evenly over meat and vegetables. Tuck fresh rosemary, thyme sprigs, and bay leaves among vegetables.

Slow Cook to Fork-Tender: Cover tightly with slow cooker lid. Cook on LOW for 8 to 9 hours (or on HIGH for 5 to 6 hours) until the beef is succulent, yielding effortlessly under a fork, and vegetables are fork-tender without turning mushy.

Thicken Rich Gravy: Carefully transfer pot roast and vegetables to a warm serving platter and tent with foil. Skim excess surface fat from cooking juices in slow cooker. Pour juices into a saucepan over medium heat; whisk in cornstarch slurry and simmer for 3-4 minutes until glossy and spoon-coating thick.

Serve & Garnish: Shred roast into large rustic chunks. Ladle velvety hot thyme gravy generously over beef and vegetables. Garnish with chopped fresh parsley and serve immediately with warm crusty sourdough.