Why does a green ring appear around hard-boiled eggs

Heat gently
Heat over medium heat. As soon as the water begins to boil, immediately turn off the heat.

Let time do its work

Cover the pot and let the eggs sit in the hot water for 9 to 12 minutes, depending on their size. For a slightly soft yolks, 9 minutes is sufficient. For a firm yolks, allow up to 12 minutes.

Express cooling
Meanwhile, prepare a bowl of ice water (or, failing that, very cold water). Plunge your eggs into it as soon as the standing time is over. This immediately stops the cooking process, prevents the green ring... and makes peeling easier!

Peel without stress

Once completely cooled (5 minutes is sufficient), peel your eggs under running cold water to easily remove the shells.