Heat 1 tablespoon olive oil in a large saucepan over medium-high heat. Pat roast dry with paper towels and season with salt and pepper.
Sear roast in pot until browned on both sides, about 4 to 5 minutes per side. Transfer roast to slow cooker.
Add remaining 1/2 tablespoon olive oil to pot. Add onion and cook 2 minutes, add garlic and cook 30 seconds longer. Pour onion mixture over roast in slow cooker.
Return pot to heat, pour in beef broth, Worcestershire, thyme and rosemary and cook about 15 seconds, just long enough to scrape up browned bits from bottom of pan. Remove from heat.
Spread the potatoes and carrots over the onion layer in the slow cooker, pour the beef broth evenly over the top, then season with salt and pepper.
Cover slow cooker and cook on low until roast and vegetables are tender, about 8 to 9 hours.
Remove roast and vegetables, shred roast (discard fat), and slice potatoes if desired.
If you want to thicken broth and make a gravy, pour broth from slow cooker through a fine mesh strainer into a small saucepan**. Heat over medium-high heat. Whisk cornstarch with 3 tablespoons beef broth, then pour into saucepan. Bring to a boil, stirring constantly; simmer 30 to 60 seconds.
Arrange roast and vegetables, pour gravy over top, and sprinkle with parsley.
