Preheat your smoker to 300°F (149°C)—you can go down lower if you like, but know that it will take longer to cook.
Fold and tie the tail under the tenderloin.
Tie the tenderloin ever 2 inches or so, to create a uniform thickness along its length.
Rub the beef tenderloin all over with pepper and salt. (We used a combination of kosher and cherry-smoked salt.)
Rub the meat all over with the olive oil.
Place the meat in the smoker, and insert the probe into the center of the loin.
Set up the air-temperature probe as well.
Close the lid on the smoker.
Set the high-temp alarm on your Smoke X4 or RFX MEAT for the meat to 125°F (52°C) for medium-rare (or 130°F [54°C] for medium). Set the air probe’s high alarm to 325°F (163°C) and the low alarm to 275°F (153°C).
Allow the meat to cook until your alarm sounds (about 1 hour).
Check the temperature with your Thermapen ONE, making sure that the lowest temperature you see is your target pull temperature of 125°F (52°C). Remove the tenderloin from the smoker, and let it rest for 10 minutes before carving. This will give the meat time for the ~5°F (3°C)carryover cooking that will occur, resulting in a perfect 130°–135°F (54°–57°C) for medium-rare or 135°–145°F (57°–63°C) for medium.
If you like, leave the probe in the meat while it rests. Smoke X4 will register the highest temperature reached before cooling as the Max temperature on the Min/Max display.
Slice the smoked beef tenderloin and serve it up.