google "flatbread"/"flatbrød" or "snapbread"/"knekkebrød" or "kavring"
It is light (you can make it high fiber for example)
It is crunchy (as kavring can typhically be made from dried bread (use a warm air oven with an open lid) and the other 2 also are just as dry, basically the durability goes up with dryness)
I myself think its tasty, especially if you put some butter on, maybe some cheese (goat cheese if you want to do it like its founding fathers and mothers)
It is long lasting, is you red my previous statement, durability goes up with dryness because of the decline/stop in bacterial growth.
It is noutrishous in the sense that you can make it contain alot of fiber, can add different kinds of seeds or grounded nuts etc. the nourishment really depends on ingrediences.
For a recipe, try gtranslating some swedish/norwegian or general scandinavian recipes, from what i know, i think the food comes from around these nations, altough got no source to prove this on hand.
Also, you can make it look like lembas bread, the leaf it comes wrapped in however wich was supposed to keep it moist does not make sense, unless it contains a desinfection liquid on microscopic level, but i guess that would violate ur query about poisons etc.
EDIT: once i red my post i kinda make myself come off as an douchebag, with spelling errors. Just wanted to include that my word usage is solely because of english not being a learned language for me, so don't assume my personality traits as a reflection of my post please.