Step 1When ready to cook, start your smoker going at 225 degrees F and preheat, lid closed, for 10 to 15 minutes.Step 2Combine all of the ingredients for the rub in a small bowl and rub liberally onto your beef roast, using your hands to press the rub into every surface of the meat. (Optional, rub your meat the night before smoking and refrigerate)Step 3Put the roast directly on your grill grate, fat-side up, and cook for 3 hours, spraying with 1 cup of the beef stock every hour (reserve the other 2 cups of stock).Step 4After 3 hours it is time to turn up the heat! Place the sliced onions in the bottom of a large disposable aluminum foil pan and pour the remaining 2 cups of stock in the bottom of the pan. Transfer the roast into the pan on top of the onions and set the pan in the grill.Step 5Increase your grill temperature to 250 degrees F, and cook until the internal temperature reaches 165 degrees F (about 3 more hours). If you're watching a thermometer, you'll notice the temperature will stay between 155 and 165 degrees for quite a while. This is called the stall period and is totally normal.Step 6Once your roast hits 165, cover the pan tightly with aluminum foil and continue cooking until an instant-read meat thermometer inserted in the thickest part of the meat registers at least 200 degrees, up to 202 degrees F (this step can take another 3 hours). Every roast will be done at a slightly different temperature, so look for your probe to slide into the meat like it is sliding into softened butter.Step 7Remove the pan from the smoker and let rest for a few minutes. Separate the roast from the cooking liquid. Shred the roast and separate the fat from the cooking liquid. Moisten the roast with the remaining cooking liquid, or make it into au jus for dipping, or turn it into gravy.