If using beef tenderloin I recommend folding the thin end piece back and tie it up with butcher's twine so the whole cut of meat is even in thickness. If you use pork loin like I do here you don't need to do anything.
Light a good amount of charcoal in your grill, or start a fire with some logs in a fire place.
Place the towel on a cutting board or directly on the kitchen counter.
Add a good 0.5" thick layer of sea salt on the towel.
Place the meat along the short side, 2 inches in. Grab the towel and start rolling it tightly. When you're a little more than halfways, fold the ends in and keep rolling.
Roll it all the way and tie it up with butcher's twine. Spray it all over with water to moisten the towel.
Place the package directly on the glowing embers. If you're using a kettle grill you can put the lid back on, but cracked open so there's a 2 inch gap to make the fire breathe properly. Leave the meat for 5-10 minutes.
Flip/rotate the meat every 5-10 minutes so it is evenly cooked. NOTE: handle it carefully so you don't crack the salt crust.
Check the meat temp after 20 minutes. For this recipe where we use pork loin the target temp is 62° C / 144° F. However, due to the carryover heat we need to remove it from the grill way before that. Remove the meat from the grill when the internal temp is 44° C / 113° F. Leave it on the cutting board to cool off for 15-20 minutes, and keep an eye on meat temp. The target temp is 62° C / 144° F.
Once the target temp is reached you crack open the package by the dinner table. Use the back of the knife handle to give it a light wack and open it up. NOTE: use a large cutting board as there will be salt and stuff from the package creating a neat little mess. Now slice the meat and serve!