Simon cooked our rare fillet of Lamb by first wrapping tightly in Glad wrap so that the fillet keeps it's nice round shape, and then vacuum sealing it.  Some people now have home vacuum sealers but I am sure if you took it to a local supermarket they would vacuum seal it for you.  Then the sealed fillet is cooked Sous Vide for 18 minutes at 61.5 degrees celsius.  Sous Vide is a french for "under vacuum" It is a method of cooking food in air tight plastic bags in a water bath, at an accurately determined temperature usually much lower than normally used for cooking in order to maintain the integrity of the ingredients. 

After the fillet has cooked for 18 min remove from plastic and it is optional to sear in a hot pan to brown the outside then rest for 10 min and slice thinly.

Home cooks are also able to use ziplock bags instead of vacuum sealing and a slow cooker with water in heated to the correct temperature using a thermometer will also work very well.  It is critical to keep watch on the temperature. 

Click on this link to watch a chef on utube cooking Sous-vide method.