Beef is cooked once it hits 58C (136.4F).
Put the stake in a 60C oven for an hour and a half, tightly wrapped in cellophane. This will ensure that the meat is fully cooked, but still rare (it's completely pink throughout). I then throw it in a hot pan for a minute on each side, and it's done. Juicy and tender.
This method is from a professional chef who cooks full fillets in this restaurant like this - when someone orders a fillet, he cuts a steak off the already-fully-cooked fillet and then just caramelises the outsides quickly in a hot pan. He'll use a thermometer to ensure that the beef is 58C before cutting it.
Other chefs use a Sous Vide to do this.