Several years ago, back when I was just landscaping and reading some Tolkien on my free time, I was reading about the use of cram, a Tolkien invention possibly inspired by the real hardtack, or ship biscuit/pilot bread made in the past for long sea voyages.
I decided to make a batch and bring it to work for the next few months, for no good reason other than curiosity.
The recipe is rather simple: it's just flour and water, with optionally a small bit of salt.
The procedure for cooking is simple as well. Preheat your oven to 400 degrees fahrenheit, and mix the flour, water, and salt into a dough. Roll it into a 1/2" thick sheet, and place on a cookie sheet. Bake for 30 minutes, remove it, and cut into 3" squares, or, as I did, use a glass to cut into circles. Then, poke some holes into the dough with a fork.
Flip the squares/circles, and bake for another 30 minutes. Remove, cool them before packaging (to let moisture evaporate), and then wrap in foil or put into a ziplock bag.
I've recently renewed some interest in this stuff, as it would be a lightweight food source for hiking, along with a protein source like textured soy protein or another dried protein.
Time will tell how well this works.