Bring a small pot of water to a boil. When the water starts boiling, add some salt and cook the eggs for 13 minutes, then cool them immediately in very cold water.
Peel the eggs and cut them in half lengthwise. Carefully remove the yolks. Finely grate half of one yolk and set it aside for garnish. Mash the remaining yolks with a fork in a small bowl.
Add the tuna, mayonnaise, salt, and black pepper to the yolks and mix well until smooth.
Fill the egg whites with the tuna mixture using a spoon or a piping bag. Sprinkle with the grated yolk and a pinch of black pepper, then garnish each egg with a small parsley leaf.
