Step 1: Pat the beef chuck roast dry with paper towels and season generously all over with salt and black pepper.
Step 2: (Optional, but recommended): Heat olive oil in a large, heavy-bottomed skillet over medium-high heat. Once hot, sear the chuck roast on all sides until deeply browned, about 3-4 minutes per side. Transfer the seared roast to the slow cooker.
Step 3: Add the chopped onion, potatoes, carrots, minced garlic, dried thyme, and dried rosemary around and on top of the beef in the slow cooker.
Step 4: Pour the beef broth over the ingredients in the slow cooker. Ensure the liquid comes about halfway up the sides of the roast.
Step 5: Cover the slow cooker and cook on LOW for 6-8 hours, or on HIGH for 3-4 hours, until the beef is fork-tender and easily shreds.
Step 6: Carefully remove the beef and vegetables from the slow cooker and transfer them to a serving platter. Tent with foil to keep warm.
Step 7: (Optional gravy thickening): If you desire a thicker gravy, skim any excess fat from the liquid remaining in the slow cooker. In a small bowl, whisk together the cornstarch and cold water until smooth. Pour this slurry into the hot liquid in the slow cooker, whisking constantly. Cook on HIGH for another 15-20 minutes, or until the gravy has thickened to your desired consistency.
Step 8: Taste the gravy and adjust seasonings if necessary. Spoon gravy over the beef and vegetables. Garnish with fresh parsley, if desired, and serve hot.