Season the beef with salt, pepper, and thyme. Heat oil in a skillet over medium-high heat and brown the roast for 3 to 4 minutes per side.
Place onion, carrots, potatoes, and garlic in the slow cooker. Set the browned roast over the vegetables.
Whisk broth, tomato paste, and Worcestershire sauce together. Pour the mixture around the roast without washing away the seasoning on top.
Cover and cook on low for 8 to 9 hours or on high for 4 to 5 hours, until the beef pulls apart easily. Transfer the roast and vegetables to a platter.
For a thicker gravy, stir the cornstarch mixture into the cooking liquid and heat until glossy. Spoon the gravy over the roast and vegetables.