Step 1 Season the tenderloin generously with salt and pepper.
Step 2 Roll the tenderloin in the pistachios, coating it completely in nuts. Wrap it tightly in plastic and let it rest in the refrigerator for 30 minutes. Remove the tenderloin from the refrigerator and bring to room temperature (allow about 30 minutes) before roasting. Heat the oven to 400 degrees.
Step 3In a roasting pan that can be used on the stove top, or in a large oven-safe skillet, heat the clarified butter and pan-sear the tenderloin, browning all sides evenly.
Step 4Roast until the meat's internal temperature is 130 degrees, about 35 minutes. Remove the roast from the oven, loosely cover with foil and let it rest for at least 15 minutes before slicing. The temperature will rise to about 140 degrees, for rare.
Step 5Serve with bearnaise demi-glace. (see related recipe)