“Garbage in, garbage out.” The line has origins in the computer programming world, but you’re just as likely to hear it in a professional kitchen. The problem is, it can be misleading—it definitely misled me for many years before I realized I was taking it too literally. Sometimes there are benefits to cooking with “garbage.” Adding onion skins to stocks and broths is a great example.
There are of course plenty of situations where cooking with “garbage” is going to be a bad thing. I’ve lost count of the number of times I’ve seen someone complain about a recipe only to realize that the problem had less to do with the recipe than their shopping choices. The real issue is trying to define what garbage even is—”one man’s trash is another’s treasure” and all that. I spent decades peeling carrots, garlic, and onions before adding them to the stockpot, and if I’m remembering correctly it was largely because I once saw a celebrity TV chef instruct a less experienced cook that peeling everything first was the better way to do it. I’m pretty sure the chef actually said something along the lines of, “You want your stock to taste like garbage? Then don’t put garbage in it,” while angrily thrusting an unpeeled onion in the cook’s face.
It wasn’t until many years later that I almost fully stopped peeling carrots in some dishes, especially roasted carrots, since I’ve come to enjoy the wrinkled skin on those. And at some point I quit peeling garlic for stock because I just couldn’t be bothered anymore, and I never noticed any difference anyway.
More recently, though, I’ve made the switch to not peeling my onions at all for most stocks and broths, unless I want a very light, clear one. See, the dried yellow onion skins get their color and an astringent flavor akin to that of black tea from polyphenols like tannins and flavonoids—antioxidants with purported health benefits (I won’t weigh in on that). Many of these polyphenols are water soluble, which means that if you put the onions with their skins in the stockpot or soup pot, you can extract the color into the broth, yielding a deeper, richer, darker brown color.
That’s a plus visually, since we do eat with our eyes and a broth that looks richer will often seem to taste richer as well (the brain works in funny ways!). But the visual effect isn’t the only reason to do this. To suss out what flavor impact if any onion skins have on stocks and broths, I steeped some onions in plain simmering water for about 20 minutes, then tasted the result. The water had an astringent, slightly bitter edge that reminded me quite a bit of strongly steeped black tea, which makes sense since black tea is also loaded with similar tannins and other polyphenols. The effect was strong in my sample because I simmered a lot of onion skin in a small amount of water. In a stockpot with a lot more water and other ingredients, it would be much more subtle.
I wouldn’t go as far as to say the simmering of onion skins in a stock or broth makes the result more delicious, but it does add a subtle depth and complexity that can complement the rich flavors of simmered meats and other vegetables. In my book that’s worth doing at least some of the time, especially when you also factor in the time savings of not having to actually peel anything.
So, the next time you’re whipping up some stock or broth, consider just chucking the onions in with their skins still on. It’s even less work, and arguably comes with a small benefit.