Remove the silverskin from the pork tenderloin, if there is any (See blog post for instructions). Halve the tenderloin to have two equal pieces.
Crust: Place the butter, herbs, shelled pistachios, breadcrumbs, and lemon zest in a food processor. Add salt and pepper to taste. Mix until the ingredients are finely chopped, and you have a rough paste. Place this mixture between two pieces of plastic wrap/cling film and press to form a small and thin rectangle, about as long as your tenderloin halves and double as wide. When the butter is cold, you should be able to cut two pieces of this mixture to cover each of your tenderloin halves. So form this rectangle to have roughly the same size as your meat.
Refrigerate: Wrap the butter rectangle in the foil and refrigerate for about 30 minutes.
Preheat the oven to 240 degrees Celsius/ 460 degrees Fahrenheit.
Season: Sprinkle the tenderloin halves with salt and pepper all over.
Sear: Heat the oil in a heavy skillet (preferably cast iron as it is oven-safe). Sear the meat pieces on all sides until nicely browned.
Top with the crust: Place the meat in a lightly oiled ovenproof dish or leave it in the skillet if that's oven safe. Take the butter out of the fridge and cut the rectangle in half. Press each piece on top of one of the tenderloins.
Roast: Place in the oven and roast for about 7 minutes. Take out and let rest for 5 minutes. Cut into pieces and serve. The meat should be cooked with just a hint of pink.