Pat the roast dry and season with the salt, pepper, and garlic powder.
Sear in the hot oil 3 to 4 minutes per side until browned.
Layer the onion, carrots, potatoes, and garlic in a 6-quart slow cooker. Set the roast on top.
Whisk the broth, tomato paste, Worcestershire, and balsamic. Pour around the roast and add the thyme and bay leaves.
Cover and cook on LOW 8 to 9 hours (or HIGH 5 to 6 hours) until fork-tender.
Move the roast and vegetables to a platter. Skim the fat from the liquid and simmer it in a saucepan.
Whisk the cornstarch and cold water, stir into the simmering liquid, and cook 2 to 3 minutes until thick.
Pull the roast into chunks and serve with vegetables and gravy.