During a trip to the battleship USS Iowa last week, we learned that the first vacuum sealer for food was developed for Naval use in 1943; same for convection ovens and soft serve ice cream machine, but I digress.

We were told that during WW II chopped lettuce and fresh tomatoes were kept fresh and useable for 3-5 months using vacuum sealing.  The part that kept my interest was that if either the lettuce or tomatoes began to show any early signs of failure, the vacuum sealed bags were put in the freezer for 10 minutes at a time, every two hours for 12 hours.  This "tricked" the food into thinking it was frozen without freezer damage and it would then stay edible for another month.   While you're spending the time cooking in quantity to put away for later use, it wouldn't be too hard to prep up the lettuce and tomatoes for a nice salad at the same time.