Cook the Meat Mixture: Heat a large pan over medium heat. Add the ground beef, breaking it apart with a spoon, and cook until browned. Remove the meat and set aside. In the same pan, add a little oil if necessary, and cook the onion, carrots, celery, and garlic until softened. Return the beef to the pan, stir in the tomato paste, Worcestershire sauce, beef stock, thyme, salt, pepper, and bay leaf. Simmer for about 20 minutes, or until the liquid has reduced slightly.