Spray or line the crock of a 5-7 quart slow cooker.
Sprinkle rub on all sides of the meat and rub in well. Place meat and onions in slow cooker.
Peel carrots and slice in half crosswise. Cut celery in 1-inch chunks. Peel potatoes. Place them all in the slow cooker.
Add the water and the stock cubes. Cover slow cooker and cook on HI for 5-6 hours or on LOW for 8-10 hours (times approximate).
HINT: I always make my pot roast a day in advance so I can chill the liquid and remove the congealed fat easily. Plus pot roast improves when it sits overnight! Try this if you don't do it already!
When roast is done, remove meat and vegetables. Either skim the fat from the liquid (or use my method of chilling to let the fat congeal for easier removal). Add the Gravy Master at this point if you are using it.
To thicken pan juices, turn slow cooker to HI and stir in 2 tbs cornstarch mixed with 2 tbs water for EACH cup of pan juices. Stir constantly until well mixed. Cook for 1 minute after juices begin to bubble.
If you cook it the day before as I do, store the meat & vegetables in a separate container from the pan juices. Skim the fat off the chilled pan juices and reheat the veggies and meat in a pan on the stove. When heated through, remove the meat & veggies and proceed with thickening the gravy, if desired, as above.
Serves about 4 people.