Heat up the cast iron pan well ahead to get it warm evenly.
Prep the steak by flattening it slightly (to make it quicker to cook and even)
Oil the meat, then place it into the hot cast iron skillet/pan.
Push it down, keep it moving, then season with salt, and brush generously with the butter and herb brush.
Once a lovely colour all over, place on a tray and into the oven. Depending on the size of the steak, it will take around 10 minutes (500g), or 2-3 (300g). Use a thermapen to measure the temperature of the steak before and after you place it in the oven to double check. You are aiming for 45c for rare, 48c-56c for medium rare-medium.
Check seasoning, give it a 'kiss' of heat in the cast iron again and carve to serve.