Pour the gravy into a slow cooker, and mix with water until smooth. Stir in half the thyme, rosemary, sage, and onion salt.
Mix the remaining thyme, rosemary, sage, and onion salt in a bowl, and stir in the olive oil to make a paste. Rub the paste all over the beef roast, and place the roast into the gravy. Place the lid on the slow cooker, and set to low setting.
Cook until the roast is very tender and the gravy is thickened, 8 to 9 hours.
