Place the beef stew meat, carrots, parsnips, potatoes, and onion into the slow cooker.
In a small bowl, whisk together the beef broth, tomato paste, thyme, bay leaves, salt, and black pepper until the tomato paste is mostly dissolved.
Pour the broth mixture over the meat and vegetables in the slow cooker.
Cover and cook on low for 8 hours or on high for 4 to 5 hours, until the beef is very tender.
In the final 30 minutes, mix the cornstarch with 2 tablespoons of cold water to form a slurry. Stir the slurry into the stew, then cover and continue cooking until the broth thickens slightly.