In a 5-qt. slow cooker, combine the potatoes, carrots and onions. Add beef; discard spice packet from corned beef or save for another use. Combine the water, marinade for chicken and pepper; pour over meat. Cover and cook on low for 8-10 hours or until meat and vegetables are tender.
Remove meat and vegetables to a serving platter; keep warm. Skim fat from cooking juices; transfer to a small saucepan. Bring liquid to a boil. Combine cornstarch and cold water until smooth. Gradually stir into the pan. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Serve with meat and vegetables.
This recipe was tested with Lea & Perrins Marinade for Chicken.