Pour enough water into the cooker to just cover the bottom. Spread the onion slices in the bottom of the cooker.
Put the pot roast in the cooker. Sprinkle the chuck roast with the flower, salt and black pepper. Sprinkle beef gravy mix, and ranch dressing mix over the roast. Distribute potatoes and carrots around the meat.
Cover the cooker, set to Low, and cook until the roast is tender and the gravy has thickened, about 8 hours.