Make your Béarnaise sauce and keep warm in a bowl covered in plastic wrap and set it to the side. This will hold for about 90 minutes.
Add 1 tablespoon of clarified butter or oil to a sauté pan until smoking and then add the potatoes and brown over medium to high heat, which takes about 3-4 minutes. move the potatoes frequently while cooking.
Add them to the oven at 350° for 30-35 minutes or until cooked through.
Garnish with parsley, butter, salt, and pepper, and set the potatoes to the side and keep warm.
While the potatoes are in the oven, truss and season the beef tenderloin well on all sides with salt and pepper.
Add the remaining 2 tablespoons of clarified butter or oil to a large frying pan or rondeau pot over medium-high heat until smoking and then add in the beef.
Turn the heat down to medium, add in the unsalted butter, garlic, and thyme and cook on all sides until very browned, which takes about 2 1/2-3 minutes per side.
Transfer the pan to the oven and cook at 350° for 15-20 minutes or until the desired internal temperature has been achieved. This amount of cooking time will render a medium-rare internal temperature at 125° internally.
Remove the beef and let rest for 3-5 minutes before slicing.
Serve the beef with the potatoes, béarnaise sauce, and chopped fresh parsley.
Notes
Make-Ahead: This beef tenderloin is meant to be served as soon as it is done resting. If you choose to make it ahead of time, the internal temperature may increase past your desire.How to Reheat: While I never recommend reheating a hunk of beef, I do realize you may not be able to eat it all. Slice the beef tenderloin into desired portions and add it a large sauté or roasting pan or with about 1 cup of beef stock, covered in foil and cooked in the oven at 400° for 10 to 12 minutes. For the Potatoes, place the desired amount on a cookie sheet tray lined with parchment paper or in a sauté pan and heat in the oven at 350° for 5-10 minutes or until hot. You can also reheat in the microwave until hot.How to Store: Place covered in the refrigerator for up to 4-5 days. The beef can also be frozen for up to 45 days covered in plastic. The potatoes do not freeze well as the potatoes will turn to mush once thawed.This is best prepared and served at a rare to medium-rare internal temperature to preserve as much flavor as possible.If you are unsure of the internal temperature, be safe and use a thermometer.While not classic, you can serve this up with a side of vegetables such as glazed asparagus.You'll only need about 1/2 the béarnaise, but feel free to use extra.