1. Cook the eggs in boiling water for 10 minutes to obtain hard-boiled eggs.
2. Remove the shells and cut them in half lengthwise.
3. Remove the yolks from the hard-boiled eggs and set aside 2. Add the drained and flaked tuna to the egg yolks.
4. Mix with a fork, mashing the egg yolks with the tuna.
5. Make a mayonnaise. Separate the egg yolk from the egg white. Add the mustard to the egg yolk and whisk while gradually incorporating the oil. Season with salt and pepper. Mix the tuna, egg yolks, and mayonnaise.
6. Arrange the halved eggs on a plate lined with lettuce leaves. Using a teaspoon, generously fill each half with the mixture.
7. Finally, grate the remaining egg yolks over the eggs.
8. Chill the dish in the refrigerator for a while and serve cold.