If the tenderloin roast is large, cut it in half cross wise to control the internal temperature. You can also save the other half (freeze) for another time. Only make what will be consumed. Heating up the tenderloin will increase its doneness. (which is okay for sandwiches).
Uncover meat. Put on a plate (blood will pull and you don't want a mess).
Season the meat with sea salt and freshly ground black pepper.
Allow to sit in refrigerator uncovered at least 12 hours. Two Days is better.
Take the meat out at least one hour (two hours is better) prior to roasting.
Preheat the oven to 425 F, 220 C, Gas 7. . Plan on about twenty to twenty-five minutes roasting, and at least ten minutes of resting time.
Sear the meat, a few minutes on all four sides.
Heat a dutch oven, frying pan or cast iron pan on medium high. Wait until the pan Is hot to add your oil.
Make sure oil is hot before placing meat in pan. Sear on all four sides.
Roast until the internal temperature reaches 125 F. , 51 C. degrees for medium rare. Remove from oven, spread the top with grass fed butter, and tent with tin foil for ten to twenty minutes.
The roast will continue to cook. The end pieces will be more well done for those who prefer that. Keep in mind that you can always put a few pieces back in the oven for those who prefer well done meat, but you can't go back and reverse it.
Generously rub with grass fed butter. You can flavor your butter with fresh thyme for extra flavor. I find thyme is best herb, complimentary to red meat.
Active Time is twenty minutes. Dry Brine time: 24 to 48 hours. Counter Time Prior to Roasting: 2 hours.