I first came across this idea many years ago from MFK Fisher's How to Cook a Wolf, but it's something I implement sort of sporadically, even though it is so sensible and wise. Last week I did it conscientiously and acquired several tasty juices that I turned into a noodle soup at the end of the week for lunch. The idea is that you save the water from whatever vegetable dishes you cook. And you can also get more vitamins out of your vegetable discards (ends of scallions, zucchini, etc. whatever you normally throw away) by giving them a quick simmer in water. So I saved the water from boiled potatoes and tossed in the odds and ends leftover from ratatouille making and cooked together for a little while and strained into a jar. I saved the jelly at the bottom of the pan from roasted chicken (fat skimmed away after refrigeration), and the juices from sauteed kale. And on Saturday these elements all came together with a piece of leftover roasted chicken, some fresh herbs and stray onions to make a noodle soup. The broth was excellent and I applauded myself for taking just a little bit of time to save these things that would normally have gone down my drain.
I'll try to be better about doing this since there are many reasonable uses for vegetable broth aside from noodle soup. A very simple soup can be made if the broth becomes concentrated enough just by swirling in an egg and chopping some fresh scallions in. Or the broth can be used to make rice or other grains.
[Out of all the things I attempted to plant on my balcony, the parsley is far and away the most successful. My two bushy plants grow about as fast as I can use it and it's wonderful snipping off a few leaves to throw on top of things. The sage on the other hand...]