We watched AB make this on his 'Good Eats' show and it looks so succulent and is surprisingly simple to make! He states: "It's difficult to get salt to stay on something other than a flat surface, so using a dough method works best".
Reserve half of the herbs for later. Mix the other half of herbs and all the other dry goods together in a large bowl. Make a well in the center and add the egg whites/water mixture.
Use a potato masher (unless you have a very strong KitchenAid) and begin to mash and mix the dough. It will be very tough and crumbly. Scrape down the sides and knead to a solid mass (it will still be crumbly).
Place dough (break up if needed) into a sturdy ziplock freezer bag. Seal up and leave on counter for 4-24 hours (this allows the egg whites to soak up the dough).
About 1 hr before dough is ready: In order to achieve uniform cooking, fold over slender tail end of tenderloin and tie with kitchen twine. Set a large electric griddle at its highest setting; brush the tenderloin with the olive oil and sear on all sides until well browned, approximately 10 minutes. Rest the meat for at least 5 minutes or until it is cool to the touch so as not to melt the dough.
Preheat oven to 400°F Transfer dough to a floured surface and roll out to 3/16-inch thickness, approximately a 24 by 18-inch rectangle. Trim away extra dough, if necessary. Sprinkle the remaining herbs on the center section of the dough and gently press down.
Place meat in the middle of the dough. Gently take one side and fold over meat. Fold back a flap. Bring up the other end and crimp together with the flap. Fold short ends over and crimp to seal. You do not want it tight around the meat, but you do want it sealed well.
Move the roast onto a baking sheet. Insert a meat thermometer into center of roast. Set to 125°F Baking time is approximately 25-30 minutes.
Remove from the oven and allow to rest for 30 minutes or up to 1 hour. The tenderloin will continue to cook 10 to 15 degrees more. Cut salt crust at 1 end and extract meat by pulling out of dough tube using tongs. Slice and serve immediately. (Alton says to toss the casing in the yard as it's good for wildlife).