Wash and pat dry pork butt. Inject 1 cup enchilada sauce and lightly coat with yellow mustard. Sprinkle on the dry rub mix. Place pork butt in 2 gallon resealable plastic bag and place in fridge overnight.
The next morning, remove pork butt from resealable plastic bag and apply a thin layer of dry rub. Fire the smoker up to 250°F and place the pork butt in with hickory wood. Keep the temperature of the smoker between 225-230°F.
Spray the pork butt with apple juice every 2 hours until the internal temperature reaches 158-190°F. Double wrap in tin foil and place the pork butt back in the smoker until the internal temperature reaches 200°F.
Wrap pork butt in a towel and place in a cooler to let the juices redistribute through the meat. An hour later, start shredding the meat and let it sit while you get the enchiladas prepped.
Heat the remaining enchilada sauce in a shallow cooking pan. Quickly dip the tortillas in the pan on both sides and add your mix of pulled pork, onions and cheese.
In a 10 x 14 inch cooking pan, rub a thin layer of enchilada sauce all over the inside.
Roll up the enchiladas and place in the 10x14 inch cooking pan. Continue the process until the pan is full. Sprinkle left over sauce, onions and cheese on top of the enchiladas and cover with tin foil.
Place the pan on the smoker rack and heat for an hour at 250°F.