In a small bowl, combine the soup mix, sugar, salt, and pepper. Sprinkle the mixture over both sides of the brisket and rub it into the meat. Place the meat, fat cap side down, into a 6 to 8-quart slow cooker. Place the thyme sprigs, garlic, and bay leaves on top of brisket. Add the carrots, potatoes, celery, and onion over top.  In a liquid measuring cup, whisk together the stock, Worchestershire sauce and cornstarch until the cornstarch is fully dissolved. Pour the lid into the slow cooker. Cover and cook on high for 5-6 hours, or low for 8 hours until the brisket is tender to slice, but not falling apart.  Remove the brisket from the slow cooker and transfer to a cutting board, fat cap side up. Remove and discard the thyme sprigs and bay leaves. Thinly slice the brisket against the grain. Serve alongside the vegetables, drizzled all over with the gravy from the slow cooker.