Preheat the pellet smoker to 225°F using oak wood pellets
Mix all of the spices (salt, pepper, paprika, garlic powder, onion powder, mustard powder, and cayenne pepper) in a bowl.
Rub the chuck roast with olive oil, then coat it with the spice mix.
Place the chuck roast on the pellet smoker, fat side up, and smoke for 3 hours. Spritz with beef stock every hour.
Slice 1 yellow onion and place in a disposable aluminum pan with 2 cups of beef stock. Place the chuck roast in the pan on top of the onions after 3 hours of smoking.
Increase the smoker temperature to 250°F and smoke the chuck roast for another 3 hours or until it reaches an internal temperature of 165°F.
Once the chuck roast reaches 165°F, cover the pan tightly in foil and cook for another 3 hours or until it reaches an internal temperature of 200°F.
Once the chuck roast reaches 200°F, remove it from the smoker and let it rest for 10-15 minutes
Remove the roast from the cooking liquid and shred or slice. Moisten the roast with the cooking liquid or turn it into au jus for dipping or gravy.
Serve and enjoy your delicious, juicy, and flavorful smoked chuck roast!