1) Use a small sharp knife to trim any silverskin from the pork fillet, then pat dry with kitchen paper. Season with salt.
2)Pour extra virgin olive oil into a large heavy-based non-stick frying pan set over a moderate-heat heat. Once smoking, sear the pork for 6 minutes, turning several times, until browned all over, then reduce heat to moderate-low and continue cooking for 3 more minutes, turning several times. The pork should reach an internal temperature of 63°C.
3) Remove from the heat, brush with butter (if using), then set aside to rest for 2 minutes. Carve, then serve.