Another surprising use? As a natural dye for fabric. Pigment-rich skins, like those in red onions, have been used to dye textiles long before artificial coloring came about. If you want to try it yourself, Anjolie Noelle, a botanical and food waste dyer, says the best way to do it is to add the onion skins to a pot of water and let them simmer for an hour before adding the fabric. Once the fabric is in the dye, let it sit for another hour.
If dyeing your own fabric is outside of your DIY-willingness, you can use the colored water to dye Easter eggs instead.