In a medium frying pan, on low heat, sear both side of the stew beef. Once it is lightly seared, place the beef on the bottom of the crock pot.
Add seasonings to the beef, and then cover with potatoes, then carrots, celery, parsnip, and onion.
Pour the beef stock and the Guinness on top of the meat and vegetables. Cover with lid, and let cook on high for about 3-4 hours, until the meat is soft.
Add in the cornstarch, stir, and let rest for about 20-30 minutes to thicken. You can lower heat and keep warm if necessary.