Here's some dead useful knowledge I've accumulated, half-deliberately and without really realizing that's what I've been doing, but hey, useful!
Basic Foods to Keep You Alive AND Healthy
(When sick, when poor, when in an apocalypse, etc.)
1. Congee. Cook rice (in milk for protein; with added meat or veggies if available/digestible) until it falls apart and you're left with goo. You can also make something-like-congee from most edible grains, and grains are one thing it's relatively easy to store in quantity. If you're poor, congee can provide you a week's worth of hot, filling meals for a couple bucks; and if you're sick or infirm, it's incredibly easy for bodies to digest and use (note: use plain rice & milk, or rice & stock/water, for sick people).
2. Stock. Food will taste better and bear more nutrients if you can cook it in stock instead of, or in addition to, water. Because stock is boiled the heck out of, it's also a safe source of water, and can be drank as a nutritive and electrolyte-providing lifesaver. Take as much water as you can boil and fill it with all the food-scraps you can get: bones, veggie trimmings, fat, you name it. (In a crisis, you throw NONE of this away; you put it all in the freezer/stockpot.) Boil for hours / until you wanna stop, strain and store/use.
3. Picklekraut has its own post, but in general, knowing how to home-ferment vegetables (which takes two ingredients to do: salt and water) can be a lifesaver for many reasons. It's a way-easier way than canning to make veggies last much longer than simple pickling, and they're all marvelously probiotic (which gut-flora can be difficult to come by on a restricted diet, or if tubs of Chobani grow scarce where you live).
4. Alcohol has a million good uses, one of which is simply that it can be sold and bartered like real money, and probably always will be — but unless you're really into it and/or have easy access to the ingredients, beer and whisky are pretty tricky to make under duress. If you can get apples (specifically squished apples/cider), home-fermenting hard cider is easy (and can be done without having a source of yeast; it will lactoferment in the right conditions, albeit slowly and with low alcohol — but enough alcohol to keep it fresh. Remember one of the first reasons we had beer was because it makes the water safe!) — and *wow* tasty; we love to do it in my house. Another option is honey: if you can make or get it in five-pound-or-so quantities, you can make mead. Yeast is tricky sometimes, but then again so is distilling — basically, I'd advise picking which is easier for you — keeping a yeast-strain on hand in the freezer and knowing how to propagate it, or building a still. (A still definitely has more apocalypse-power, but fermenting stuff is a great home skill.) Even just knowing how to produce alcohol (and both fermenting and distilling take practice) is pretty valuable in some circumstances.
5. Sprouted seeds can save your buuuuutt: They're virtually free, fresh veggies that you can grow indoors and turn around into food in a matter of days. The seeds can be stored a long, long time before sprouting, and once they're sprouted they can be everything from "just eaten by the handful" to "fried up extravagantly with butter and spices" to "made into flour". *Sprouted* seeds (unlike non-sprouted ones) have protein, particularly bean and legume seeds, and don't need to be cooked, meaning they retain all the vitamins of raw veggie-food, which can be hard to get sometimes. They're also ridiculously delicious, and if your diet is limited or your'e subsisting on rations, fresh greens can be amazing. They require about $10 in equipment (plus seeds to sprout), a little reading and some counter-space, and everyone should know how to do them.
If you know of more foods (or related things) like this, I'd love to hear 'em!