Pat and season the roast. Blot the chuck roast dry with paper towels.
Season all over with 1 1/2 teaspoons kosher salt and 3/4 teaspoon pepper.
Sear for flavor. Heat olive oil in a large skillet over medium-high. Sear the roast 3–4 minutes per side until well-browned. This builds a flavorful crust and richer pan juices.
Layer the vegetables. Add onions, carrots, and potatoes to the slow cooker.
Sprinkle with the remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Place the bay leaves on top.
Make a quick braising sauce. In a bowl, whisk beef broth, tomato paste, Worcestershire, Dijon (if using), thyme, rosemary, and garlic. Pour it over the vegetables.
Add the roast. Nestle the seared roast on top of the vegetables so the juices flow down as it cooks.
Cook low and slow. Cover and cook on LOW for 8–10 hours or HIGH for 4–6 hours, until the roast is fork-tender and easily pulls apart.
Low gives the best texture.
Rest and shred. Transfer the roast to a cutting board and rest 10 minutes. Shred or slice against the grain.
Thicken the gravy (optional). Skim extra fat from the cooking liquid. Whisk cornstarch and cold water to make a slurry.
Stir into the slow cooker juices, cover, and cook on HIGH 10–15 minutes until slightly thickened.
Serve. Spoon vegetables onto plates, top with beef, and ladle over gravy. Finish with chopped parsley for color and freshness.