Remove from fridge 30 minutes before cooking and allow to come to room temperature.
Heat some butter in a pan and brown the chateaubriand on all sides until browned.
Transfer to an oven preheated to 200ºC/Gas Mark 6 for:
15 minutes for rare
17 minutes for medium
20 minutes for well done
After cooking, cover and leave to rest in a warm place for 15 minutes.