Is there any advantage to leaving the skins on onions when making stock?
To determine what effect onion skins might have, we made two batches of simple chicken stock, one with peeled, chopped onions and the other with onions we chopped and added straight to the pot, papery skin and all. The two stocks looked quite similar as they cooked, but when they were strained, we noticed a marked difference in appearance: The stock made without skins was pale yellow, while the stock made with skins had an appealingly deep golden-brown color. But that difference turned out to be superficial: Tasters agreed that both stocks tasted virtually identical.
THE BOTTOM LINE: Chopping your onion, skin and all, is a nifty timesaver, and it can give richer, appealing color to stock or gravy, but it won’t affect flavor.