Season the beef cheeks well with the BBQ rub. Allow the seasoning to sit on the meat for a few minutes before you smoke them.
Put a probe into the largest cheek section and set the high-temp alarm to 160°F (71°C). Smoke the cheeks! This step will take about 2–2-1/2 hours.
When the beef gets up near the target temp, heat the beef broth.
When the meat is fully at temp, place it into the pan with the onions and garlic and add the hot broth. Cover the pan tightly with foil.
Insert a probe through the foil and set your high-temp alarm for 203°F (95°C). Put the pan back into the smoker and smoke to temp (about 4 more hours).
When the high-temp alarm sounds, check that it is at the proper temperature throughout and that it is probe-tender using your Thermapen ONE.
Allow the meat to rest in the pan for at least 15 minutes, during which time you can heat your tortillas and prepare your toppings.