1. Pat the beef chuck roast dry with paper towels, then season all over with kosher salt and black pepper.

2. Heat the olive oil in a large skillet over medium high heat. When the oil is hot, sear the roast for 3 to 4 minutes per side, until deeply browned. Transfer the seared roast to the slow cooker.

3. In the same skillet, add the sliced onions and a pinch of salt. Cook over medium heat, stirring often, until the onions are softened and starting to turn golden, about 8 to 10 minutes.

4. Stir in the minced garlic and cook for 30 seconds, just until fragrant. Add the tomato paste, Worcestershire sauce, Dijon mustard, dried thyme, and dried rosemary, stirring to coat the onions.

5. Pour in the beef broth and white wine (or extra broth), scraping up any browned bits from the bottom of the pan. Bring the mixture to a gentle simmer, then carefully pour it over the roast in the slow cooker.

6. Cover and cook on low for 8 to 9 hours, or on high for 4 to 5 hours, until the beef is very tender and easily pulls apart with a fork.

7. Transfer the cooked roast to a cutting board and shred it into large chunks, discarding any large pieces of fat. Skim excess fat from the surface of the cooking liquid in the slow cooker if desired.

8. In a small bowl, whisk together the cornstarch and cold water to make a slurry. Stir this slurry into the onion and broth mixture in the slow cooker, then add the butter. Cover and cook on high for 10 to 15 minutes, or until the sauce thickens into a glossy gravy.

9. Return the shredded beef to the slow cooker and gently stir to coat the meat in the French onion gravy. Taste and adjust seasoning with extra salt and pepper if needed.

10. Sprinkle with chopped fresh parsley and serve the French onion pot roast hot over mashed potatoes, egg noodles, rice, or toasted bread, spooning plenty of onions and gravy over each serving.