Most modern military rations actually date back to the canning revolution of the lat 1700s and early 1800s.
The panforte (excellent reference BTW), hard cheese, crusty breads and other "semi-preserved" foods are about the only real historical foods that could withstand long term storage. Most grain was uncooked, but cracked and stored for long periods, the only problem was the possibility of grain weevils. After a few weeks, they became "extra protein" (It's amazing what you can get used to eating in the field.)
As stated earlier, water was right out, it went bad or stale too quickly and harbored all sorts of wee beasties. Alcohol was all that was safe. Ales, beers, wines and eventually mead and rum were all sourced of liquid refreshment for the troops. No wonder infection was rampant when the troops were wounded, their bodies immune systems must have been soused.
Nuts would have been a great ration, if they were available, however, they were usually so expensive only the rich could afford them. So a knight might have them, but a ranger or barbarian - uh-uh (unless of course they stole them from restricted forests, which would mean death by the King's noose or axe.)
Berries would be great for adventurers in the field, assuming you knew what was safe to eat and what was not (deadly nightshade looks a lot like very small blue berries unless you know what you're looking at and while pine nuts are safe to eat, but most coniferous berries are not. But as for "iron rations" they probably weren't part of the staple.
Gruel, a thick pasty oat, barley or wheat mixture (oatmeal, farina and barley paste) boiled in water is probably the majority of what you would find outside of dried food or salted anything.
