Will slow cooking (for 12 hours+) at 50 - 51 degrees C, after searing the steak, make meat more tender?
Nigella's Vodka-Marinated Steak (from Feast and on the Nigella website) uses a rump steak (in the US this is a sirloin steak) that is marianted for 2-3 days. Rump steak is a less tender cut of meat than fillet or sirloin (US NY strip/top loin) steak so the vodka marinade helps to tenderize the meat, as well as adding flavour.
The steak is then seared and cooked briefly in a hot frying pan so that the meat is medium-rare. After resting the meat is cut across the grain. The marinating, cooking and cutting all help to make the steak as tender as possible. We would not particularly recommend slow-cooking the steak after searing as it is likely that the steak will be fully cooked and even though it may not be that tough it is likely that it will be slightly dry as this cut of steak is not that well-marbled with fat. It may be possible to cook the steak using sous-vide techniques but unfortunately we have not tried these techniques and also we suspect the timings would be shorter than 12 hours.