Blend garlic and serranos using a small food processor, then mix into softened butter. Add 1 tablespoon of pork rub and mix to combine.
Trim excess fat off of pork loin, if necessary, mine didn't need it. Make sure the pork is at room temperature, then make slits all over the roast and fill them with the compound butter mixture (see pro-tip). Smear any leftover butter all over the pork loin, or use olive oil. Season generously with pork rub.
Clean smoker, if needed, then add pellets. Ignite smoker and preheat to 200°F, select super smoke. When smoker is ready, place pork directly on grill grates and insert probe into the thickest part. Set the probe's final temp to 145°F.
Spritz pork loin with apple juice, and again every hour. After 5 hours, turn the temperature up to 225°F, if it's done sooner, disregard. Continue to smoke the pork loin until the internal temperature reaches 145°F. Mine took a total of 6 hours.
Remove pork loin and place on a baking pan. Tent with foil and rest for 15 minutes before slicing and serving.
Notes
Here's the idea that came to me after I did this recipe the hard way... Roll compound butter in plastic wrap and freeze, then slice into small cubes and insert into the slits. It will be much easier to do then working with softened butter!
At this point, you can wrap the roast in plastic wrap and store it in the fridge or go straight to the smoker!
My pork loin didn't have much of a fat cap so it didn't need to be trimmed. However, if yours does, just trim off the excess.
When seasoning the pork loin, sprinkle the rub without touching the meat to avoid cross-contamination. You'll want to save the remaining rub for another use.
My boneless pork loin weighed 6.25 lbs. If yours is smaller or larger the smoking time may need to be adjusted.
If the roast is cold, the butter will not stick to it, it will stick to your hands instead. If the butter clumps, no problem! Just smooth it out with a pastry brush after the first hour of smoking, before spraying it with apple juice.
Don't skip resting the pork loin. This allows the juices to redistribute throughout the roast, resulting in a juicier roast!