To ensure a beef roast turns out as tender as possible, cook it in a slow cooker on low heat for 10 to 12 hours. Add carrots, onions and celery to the beef, and then add potatoes six hours into the cooking process to make a savory pot roast.
Cut the beef roast, 1 pound of peeled carrots, 1 pound of celery and 2 peeled onions into pieces. Place the roast and vegetables in the slow cooker. In a bowl, add 2 packets of onion soup mix, a 10.5-ounce can of cream of celery soup base, a 10.5-ounce can of cream of mushroom soup base, 6 tablespoons of all-purpose flour and 5 small beef bouillon cubes. Fill one of the soup cans with water, and add it to the mixture in the bowl. Do this once more, and sprinkle 2 teaspoons of garlic powder and 2 teaspoons of ground pepper over the mixture.
Combine the mixture evenly, and pour it over the roast and vegetables. Turn on the slow cooker, and set it to low heat. Allow the roast to cook for six hours. Peel 3 to 4 russet potatoes, and slice them into pieces. Add them to the slow cooker, and cook the pot roast for an additional six hours.