Turkey gravy should be intensely rich and savory, just thick enough to coat a slice of turkey, and brown in color. Color does not always indicate flavor, but there is something wrong about white gravy on white meat. I like my gravy to have contrast, especially when served as part of a highly-photographed meal—and nothing brings the brown like onion skins.
Onion skins are highly pigmented, so much so that they’re often used as a natural dye. When added to stock, they impart a deep amber color, but not too much flavor. To see how much color they brought to the stock pot, Cook’s Illustrated made too different batches of a simple chicken stock: One made with skin-on onions, and one made with peeled onions. The stock made with the skin-on onions was much darker, which you can see by clicking through to the full article. But in the case of turkey stock, which will go on to be turkey gravy, I like to take things even further by adding extra onion-less skins to the pot. (The onions must be tan or brown in color—red onion skins will not work for our purposes.)
There’s really not much to it: Every time you peel an onion, set the skins aside, then place them in a freezer bag and store them in the freezer until you’re ready to make stock. Chuck them in the pot, along with your peel-on onions (cut off the root if it’s dirty), and make your stock however you normally do. Drain out the skins, along with all the bones and vegetable matter, then use that dark and beautiful liquid to make dark and beautiful gravy.