Sous-vide is a method of cooking foods at low temperatures in a vacuum sealed bag. I read Serious Eats blog and they had an article on how to do this at home low tech style:
Another good reference is: http://amath.colorado.edu/~baldwind/sous-vide.html
I was shopping at a 99 Ranch store (that store could be another topic) and they had whole beef tenderloin (also known as filet mignon) for $3.99/lb. I grabbed a fairly small one at little over 4 lbs. That is a fantastic price for beef tenderloin.
I seasoned using a pepper based rub and placed in a Ziploc bag. Using a straw, I sucked the extra air out of bag for vacuum. I cooked this in a small cooler with water temp starting at 145. Medium rare is 135, but I know meat would lower temp. After 30 minutes, water temp was about 130, so I added some boiling water to cooler and brought up to 138. I let this sit in water bath for 3.5 hours while watching Cowboys/Texans game. The water dropped 1-2 degrees per hour probably because I used a crappy cooler.
When I removed outer meat color was a brownish grey, but slicing into meat revealed perfect cooked medium rare from edge to center. I cut into 1.25" slices, seasoned lightly, and then seared these on cast iron pan for about 30 seconds each side. The end results was very good. Served with some OGD 114.