Heat 1 Tbsp olive oil in a large skillet over medium-high heat. Season the beef chuck roast with salt and black pepper. Sear the roast until browned on all sides, about 4-5 minutes per side. Transfer to the slow cooker.

Add remaining 0.5 Tbsp olive oil to the skillet. Add sliced onion and cook for 2 minutes, then add minced garlic and cook for an additional 30 seconds. Pour onion and garlic mixture over the beef in the slow cooker.

Deglaze the skillet with beef broth, Worcestershire sauce, thyme, and rosemary. Scrape up browned bits from the bottom, then pour this mixture over the beef in the slow cooker.

Place potatoes and carrots on top of the beef. Pour the remaining beef broth evenly over the vegetables. Cover and cook on low for 8 to 9 hours until the beef is tender.

Remove the beef and vegetables from the cooker. Shred the beef if desired. If you want to make gravy, strain the broth into a saucepan, heat over medium-high, whisk in the cornstarch mixture, and simmer until thickened, about 30-60 seconds.

Serve the shredded beef and vegetables with gravy poured over the top. Garnish with chopped parsley.