UPDATE in OP: Costco Prime Tenderloin Roast, Yeah or Nay?

The Chateaubriand cut is great, but just one part of the whole tenderloin. Salting and letting it sit uncovered for a day or two isn't about tenderizing it, it's a way to dehydrate the outside of the roast, like a quick dry-aging process. This means the outside will brown quicker. This article from Serious Eats explains it more scientifically:


You've Got to Dry to Get Brown

The first step to better browning is to realize that wet things don't brown. Because water evaporates at 212°F (100°C), until you've fully desiccated the surface of a piece of meat, it's very difficult to get it to rise beyond that temperature. On the other hand, browning reactions don't really take place in earnest until temperatures reach into the 350°F+ range (177°C and up).

Knowing this, I decided to pretreat my meat in two different ways: salting heavily and a lengthy rest.

Salting a piece of meat does more than give it flavor. It intrinsically alters its muscle structure. As salt dissolves in meat juices and works its way slowly into the meat, it dissolves a protein called myosin—one of the proteins responsible for the shrinkage that occurs when meat is heated. Visually, this change is quite apparent when you compare a piece of salted meat to a piece of fresh meat. Denatured proteins scatter light in a different way from intact ones, giving salted meat a deeper red, slightly translucent appearance, kind of like ham.

A piece of meat that is salted for a prolonged period of time will shrink less and expel less moisture as it cooks. Less moisture expelled means less moisture to evaporate, which means more efficient browning. Since a thick roast can't really be seasoned internally until it's sliced and served, I like to salt my meat quite heavily around the edges.

Resting the meat after salting offers its own obvious advantage: partial dehydration of the exterior. As I talked about in this piece on dry-aging (or not, as the case may be), a steak that's been left to sit uncovered for a night or two on a rack in the fridge will develop a nice dry pellicle that will brown very fast. Combining the salting step with an overnight rest led to a roast that was extremely dry on the exterior after its initial slow roast.