I would say beans as long as it was all varieties of beans. I grow black beans and great northern beans. They are picked just when the pods are starting to turn color, but some seem to get into my bucket that cannot be shelled. They are snaped and processed with the shelled beans.Some kind of bean would seem to be a good possibility, perhaps even peas. In their younger stages, some varieties have an edible pod. Or developed more fully, a good sized green pea or bean. If left to mature to near the seed stage, it might be possible to get flour from the seeds/peas/beans.
Only one veggie is tough. There are just so many good ones.
What are you mainly thinking of? Are you thinking of something to grow for market? Or something to grow for calories? Or something to grow for nutrition? Or something you can grow very large quantities of with minimal input? Or some other reason? Just curious.
A lot depends on WHY we're only growing one vegetable. I did find that after having 82 tomato plants on purpose and maybe 3 volunteers that I didn't have the heart to yank up (which did give me some very nice cherry tomatoes and a few nice plum tomatoes, BTW), I did find that even the locally grown farm stand tomatoes just couldn't hold a candle to the tomato flavor I was able to pick right off the vine. I am now completely spoiled.Tomatoes,because I can buy so many other veggies very cheeply.
Well, if a person was going to have to produce the most calories in a small area. Or if there was another "year without a summer" where only a few crops were able to grow enough to make a crop. Or a person was sick and unable to do much, then a crop like potatoes can produce a lot for little effort.What scenario is making me grow one alone? Aint gunna happen!!! lol.