Preheat the smoker to 225° Fahrenheit.
Butterfly the pork loin to create a flat surface for stuffing.
Season the inside of the butterflied pork loin with salt and pepper.
In a mixing bowl, combine breadcrumbs, chopped spinach, feta cheese, dried cranberries, garlic, rosemary, and thyme. Add chicken broth and mix until the stuffing is moistened. Season with salt and pepper.
Spread the stuffing evenly over the inside of the pork loin.
Roll up the pork loin tightly and secure it with kitchen twine.
In a small bowl, combine brown sugar, paprika, garlic powder, and cayenne pepper to make the rub.
Rub the vegetable oil over the surface of the pork loin, then apply the spice rub.
Place the stuffed pork loin in the smoker. Smoke until the internal temperature reaches 145° Fahrenheit, approximately 2-3 hours.
Remove the pork loin from the smoker and let it rest for 10 minutes before slicing and serving.