Remove the fat and the membranes from the tenderloin. Cut out the sirloin and cut into four more or less equal portions. Sprinkle the tenderloin with salt and pepper and grill it over the coals to 48-51 °C core temperature. Do not overcook otherwise it will be tough, do not go beyond medium-rare. About 3 minutes per side. Remove the tenderloin from the barbecue and let rest under aluminium foil. Squeeze the air supply of the barbecue to reduce the heat. Place a cast iron pan on the barbecue and heat the oil in it. Add the garlic and anchovies and cook until the garlic is golden brown and the anchovies have dissolved in the oil, about 3 minutes. Melt the butter in the oil and then stir in the lemon juice. Remove the pan from the heat. Put the tenderloin in the pan and spoon the sauce over it. Cut it perpendicular to the thread. This way, you get short fibres in your mouth which feel more tender than long muscle fibres. Serve immediately, spoon the sauce over the meat and garnish with the parsley.