Pat the chuck roast dry with paper towels. Season all over with half the salt and half the pepper. Let it sit while you heat the pan.

Heat a heavy skillet over medium-high heat and add the vegetable oil. When it shimmers, sear the roast on all sides until nicely browned (2–3 minutes per side). Transfer the roast to the slow cooker.

Reduce heat to medium and add the butter to the skillet. Add the sliced onions, remaining salt and pepper, and sugar if using. Cook, stirring occasionally, until the onions are soft and golden (8–12 minutes).

Stir in the minced garlic and cook 30–60 seconds until fragrant.

Pour in the white wine (or extra broth) and scrape up the browned bits from the pan with a wooden spoon. Let the liquid reduce a minute.

Transfer the onion mixture into the slow cooker on top of the roast. Add beef broth, Worcestershire sauce, Dijon (if using), thyme, and bay leaf.

Cover and cook on LOW for 8–10 hours or on HIGH for 4–5 hours, until the roast pulls apart easily with a fork.

Remove the roast to a platter and tent with foil to rest for 10–15 minutes. Discard the bay leaf and thyme stems if used.

If you’d like a thicker gravy, whisk the flour into the cold water to make a slurry. Stir the slurry into the slow-cooker juices and heat on HIGH for 10–15 minutes until thickened. Taste and adjust seasoning.

Slice or shred the roast, spoon onions and gravy over the meat, and garnish with chopped parsley.