Bachelor Chow (See: Futurama). It doesn't have to be particularly tasty, but a simple meal that has 1/3 of your daily nutrition (with 1/3 RDA of all your vitamins and minerals) would be great. Preferably without ridiculous quantities of preservatives and artificial flavours.
There's Clif bars. You can't live off of those full time, though. Colcannon? If this is made with turnips, kale, and lots of buttermilk, then it covers a lot of macronutrients. Then take a multivitamin.
When we ran out of the box, I went back for my Clif Mojos and found out they were recalled in the peanut salmonella thing. It looks like I missed eating potentially contaminated food by a few days.