1. Pat the chuck roast dry and season it all over with salt and black pepper.

2. Heat olive oil in a large skillet over medium-high heat and sear the roast on all sides until browned.

3. Add butter to the skillet, then cook the sliced onions until softened and lightly caramelized. Stir in the garlic and cook briefly.

4. Transfer the onions and garlic to the slow cooker. Pour in the beef broth, French onion soup mix, and Worcestershire sauce. Add the thyme and bay leaf.

5. Place the seared roast into the slow cooker and spoon some of the onion mixture over the top.

6. Cover and cook on low for 8 to 9 hours or on high for 5 to 6 hours, until the beef is fork-tender.

7. Remove the bay leaf and thyme stems. Transfer the roast to a platter and shred or slice it.

8. Mix the cornstarch and cold water in a small bowl, then stir it into the cooking liquid if you want a thicker gravy. Let it cook for a few more minutes until slightly thickened.

9. Spoon the onions and sauce over the pot roast, then finish with chopped parsley before serving.