Add water, vinegar, and onion skins to a wide saucepan. Stir together. (See notes)*
Bring to a boil, cover, and reduce to simmer for 30 minutes. Remove from heat, uncover, and let cool for another 30 minutes. While the dye cools, carefully wipe the eggs with a wet cloth to remove any dirt or anything that's clinging to the shell (See notes)*
Add the eggs in a single layer, carefully placing them in between the onion skins (you may need to remove some of the skins to make room, which is fine. If the eggs aren’t completely submerged in the dye, add in a cup of cold water.
Bring to a boil, then cover and reduce to simmer for 8 minutes.
Remove from heat and let cool on the counter for 1-2 hours, uncovered, checking periodically to see if the eggs have reached your desired shade of red. Move to fridge overnight if the color isn’t to your preference (I left mine in the fridge for about 16 hours to achieve the deep red color shown in the photos).
Using a slotted spoon, carefully move eggs to a plate or rack and let air dry. (See notes)*
Lightly polish with olive oil (or any edible oil) to give it shine.
Keep stored in the fridge until ready to use (they can be kept about a week).