If your criteria for a perfect steak require the meat to be evenly cooked from the centre to edge, with a tasty char on the outside, then conventional methods will not deliver that. But sous-vide cooking can. This steak is deliciously tender, and the black peppercorn sauce is fabulous too.
You want your steaks to be as thick as possible - at least 2-3 centimetres (an inch or more). The exact cut is down to you, but even cheaper cuts will deliver good results.
Add warm water to your water bath and heat it to 54.5°C (130°F). This temperature will give you medium-rare steak - if you want a different level of doneness, a higher or lower temperature will give you that, and there's plenty of reference material on the Interwebz.
Season the steaks with salt and pepper pressed into both sides and pop them in a zip-lock bag or vacuum pouch along with the butter. If using a vacuum sealer, seal the bag and place it in the water bath, otherwise close the zipper on the zip-lock bag but leave a few centimetres open for the air to escape. Immerse the zip-lock bag in the water bath, make sure all the air is expelled and then fully close the zipper. Leave to cook for 2 hours.
Add the red wine reduction, beef stock and mustard, stir well and simmer gently for about 10 minutes. Just before you are ready to use the sauce, stir in the cream.
Stir any juices from the sous-vide bag into the sauce and pour it around the steak.