Let the tenderloin come to room temperature for at least an hour
Preheat the oven to 475 degrees F
Fold the thin ends of the tenderloin against the rest of the roast and use kitchen twine to tie it off every inch or so to hold it together.
Place the tenderloin on a roasting rack with the potatoes under the beef. Season the tenderloin generously with salt and press the crushed peppercorns all over the surface of the meat, making sure to coat all sides. Coat the potatoes in salt, pepper and olive oil. Insert a meat thermometer into the tenderloin and transfer into the preheated oven until the beef registers 120 to 125 degrees F for medium-rare/rare, about 20-25 minutes.
While the meat is in the oven, melt the butter with the garlic in a small skillet, and allow the butter to slightly brown.
Once the butter is set aside, add all the chimmichuri ingredients to a food processor and pulse until smooth. Taste and adjust seasoning as needed.
Remove the meat once it'c cooked and pour the garlic butter over immediately. Cover the meat loosely with foil and allow to rest for 10 minutes before slicing. Slice the tenderloin into 1/2 inch pieces, and transfer to a serving platter with the roasted potatoes and drizzle with the chimmichuri sauce. Serve immediately.