Cut brisket in half across the grain and place a half into a slow cooker. Spread with half the ketchup and grape jelly; sprinkle with half the dry onion soup mix. Place remaining piece of brisket on top of the first piece and repeat spreading with remaining ketchup, grape jelly, and onion soup mix. Sprinkle black pepper into cooker.
Cover the cooker and cook on Low until meat is very tender, 8 to 10 hours.