Pat the chuck roast completely dry with paper towels on all sides — this is essential for a proper sear. Season generously all over with kosher salt, black pepper, and garlic powder, pressing the seasoning in firmly.

Heat the vegetable oil in a large cast iron skillet or heavy-bottomed pan over high heat until shimmering and just starting to smoke. Add the chuck roast and sear without moving for 4 to 5 minutes per side, including the short ends, until a deep mahogany crust forms on all surfaces. Transfer the seared roast to the slow cooker insert.

Reduce the skillet heat to medium. Add the onion chunks and smashed garlic to the same pan (do not wipe it out — those browned bits are gold). Cook for 2 minutes, stirring occasionally, until softened and fragrant.

Add the tomato paste and stir into the onions and garlic. Cook for 1 to 2 minutes until the tomato paste darkens slightly and begins to stick to the pan.

Pour the beef broth into the skillet and use a wooden spoon to scrape up every browned bit from the bottom of the pan. Add the Worcestershire sauce and soy sauce, stir to combine, and bring to a brief simmer. Remove from heat.

Arrange the carrot chunks and halved potatoes on the bottom and sides of the slow cooker insert around the seared roast.

Pour the braising liquid from the skillet around (not over) the roast. Tuck the thyme sprigs, rosemary sprigs, and bay leaves into the liquid around the beef.

Place the lid on the slow cooker and cook on LOW for 8 to 10 hours, or until the beef is completely fork-tender and falls apart easily when pressed. Do not cook on HIGH if you want the best texture — low and slow is non-negotiable for true tenderness.

Carefully transfer the cooked beef to a cutting board and the vegetables to a serving platter. Remove and discard the thyme sprigs, rosemary sprigs, and bay leaves.

Pour the braising liquid from the slow cooker into a medium saucepan. Bring to a simmer over medium-high heat.

In a small bowl, whisk together the cornstarch and cold water until completely smooth to form a slurry. While stirring the simmering braising liquid, slowly pour in the cornstarch slurry. Continue to simmer, stirring frequently, for 3 to 5 minutes until the gravy thickens to a glossy, coating consistency. Stir in the butter if using, taste, and adjust salt and pepper.

Using two forks or your hands, gently pull the chuck roast into large, rustic chunks — it should fall apart effortlessly. Arrange the beef on the serving platter with the vegetables, ladle the gravy generously over everything, and garnish with a few fresh thyme leaves. Serve immediately with crusty bread or creamy mashed potatoes for soaking up every drop of that gravy.