Preheat smoker to 225°F. Mix rub ingredients in bowl and rub outside the chuck roast.
Place the chuck roast on the grates, fat-side up. Cook until the internal temperature reaches 165°F, about 3 hours. Spray every hour with 1 cup of beef broth.
Transfer the roast to a disposable aluminum foil pan and add the remaining 1 cup beef broth and sliced onions. Cover tightly with foil and cook until the internal temperature reaches 202°F.
Remove the beef from the liquid and let rest for 15 minutes before shredding. Strain the fat from the liquid, and add the meat back into the juices or reserve for a dipping sauce.
Notes
Use the right wood for smoking. Experiment with various wood chips or chunks to add distinct flavors to your pulled beef. Hickory, mesquite or oak are popular choices.
This is one of those recipes where you cook to temperature, not time.
Speaking of temperature, I recommend a good instant read probe thermometer so you can track throughout the smoking process.
To reheat, place the beef that is in the freezer bag in a pot of simmering water (not boiling) for about 20 minutes. This lets the beef keep its juices.