Note, you can season the beef and then cover and refrigerate it overnight or you can cook it right after seasoning.
If you want to season and cook it right away, make sure to let it warm up on the counter for 30-45 minutes first, before seasoning.
Prepare the beef:
Place beef chuck in a shallow rimmed baking pan and liberally sprinkle beef rub all over it. Pat it in and turn over to season the other side. Make sure that the whole beef chuck is seasoned well, on all sides.
Before placing it on the smoker, make sure to insert a leave-in thermometer probe through the side and towards the center of the beef.
Smoking Beef Chuck:
Prepare your smoker according to your smoker’s needs and don’t forget to add the wood! Preheat the smoker to 225° and place seasoned beef chuck onto the smoker.
Close the lid and let it smoke for about 3 hours (when internal temperature will reach around 165°). Maintain the smoker temperature about 225° but no higher than 240° during this smoke.
Spread sliced onion in the bottom of the skillet (make sure the skillet is large enough) and pour in the mixture of beef stock, apple juice, and a tablespoon of beef rub seasoning.
Take the chuck off the smoker and place it on top of the onions. Cover the whole skillet with aluminum foil, nice and air-tight. (Make sure not to take out the thermometer probe.)
Place the skillet with the beef back onto the smoker, raise the temperature to 250°, and let it cook until the internal temperature reaches 203°-205°. (It could take 3-5 more hours, depending on the size and the beef itself.)
Rest and Pull:
Take the beef off the smoker and let it sit on the counter and rest, still covered, for about 20-30 minutes.
Take the foil off and use meat claws to shred the beef.
Tip: if you have a lot of excess juices, you can pour some out and use it in sauce.