1 1/2 to 2 pounds kosher salt
6 to 8 sprigs oregano, rosemary, or thyme
1 center-cut trimmed beef tenderloin (2 to 2 1/2 pounds)
1 recipe ají or chimichurri, if desired
Dip a clean dish towel in a bowl of cold water or run it briefly under a cold tap, then wring it thoroughly so it is just slightly damp.
Lay it on your work surface, with the short edge parallel to the counter edge. Spread a layer of salt at least 1/2 inch thick on the towel, starting about 1 inch up from the leading edge and progressing back about 12 inches, leaving a 2-inch space on either side.
Scatter with herbs and place beef directly on the salt on the edge closest to you.
Carefully roll the beef up until it is completely surrounded in salt. Tuck the leading side of the towel into the roll, fold in the sides to form a neat package, then continue rolling until the beef is completely wrapped in the towel.
Using butcher’s twine, secure the package at 1- to 2-inch intervals, tying as tightly as possible.
Light one chimney full of charcoal. When all the charcoal is lit and covered with gray ash, pour out and arrange the coals on one side of the charcoal grate.
Place beef directly on coals. Partially cover grill and cook for 10 minutes.
Flip beef and continue cooking until an instant-read thermometer inserted into the deepest part of the package registers 95°F for rare or 105°F for medium-rare. (I do not advise cooking for longer than that if you want juicy meat.)
Transfer the whole package to a metal tray or pan and let rest until the core reaches 120°F for rare or 130°F for medium-rare, about 10 to 20 minutes.
When ready to serve, carefully crack open the crust with the back of a knife, reserve some of the smoky salt for serving, and discard the burnt cloth and remaining salt.
Use the back of your knife or a pastry brush to remove any excess salt. Slice tenderloin and serve with ají or chimichurri as desired.