Crush the pistachios in a blender or food processor. Place on a shallow plate.
Pat the tenderloin very dry and season generously with salt and pepper.
In a small bowl, combine Dijon mustard and honey. Brush a thin layer onto the tenderloin.
Press the crushed pistachios all over the surface of the meat, making sure to coat all sides. Wrap it tightly in plastic wrap and refrigerate for 30 minutes.
Remove the beef tenderloin from the fridge and set it aside.
Preheat the oven to 450°F (232°C).
Line the baking sheet with aluminum foil. Place the baby potatoes on it. Coat them with salt, pepper, and olive oil. Place the tenderloin on top of the baby potatoes.
Insert a meat thermometer into the tenderloin and transfer it into the preheated oven. Roast for 10 minutes. Reduce heat to 275°F and continue to roast until the meat's internal temperature is 135°F (for medium-rare), about 30-40 minutes.
Remove the roast from the oven. Loosely cover with foil and let it rest for at least 15 minutes before slicing. Carve into slices and serve with roasted potatoes and chimichurri sauce.