Add the onion skins to a medium-sized cooking pot.
Pour the 2,5 liters of water and the 200 ml of vinegar into the pot. Add the salt and bring to a boil. Reduce heat to medium and simmer for about 40 minutes. Remove from heat and set aside for 10-15 minutes to cool slightly.
Wash the eggs well and dry them with a kitchen towel.
With a tablespoon, dip the eggs carefully one by one into the dye. Once you have added all the eggs into the pot, transfer the pot to the heat again and simmer over medium heat for about 30 minutes.
Remove from heat and let the eggs rest in the pot for at least half an hour. Transfer the eggs to a plate and let them dry. Do not rub the eggs while they’re still wet.
Add some olive oil on a paper towel and rub each egg gently until it gets shiny all around.