Hard boil the eggs:
Place the eggs in a pot and cover them with cold water. Bring to a boil over medium-high heat. Once boiling, reduce the heat to low and simmer for 10-12 minutes. Remove the eggs from the pot and let them cool in a bowl of ice water or under cold running water.

Prepare the filling:
While the eggs are cooling, drain the tuna and place it in a medium bowl. Use a fork to flake the tuna into small pieces. Add the mayonnaise, Dijon mustard, sweet pickle relish (if using), lemon juice, salt, and pepper. Mix everything together until well combined and creamy.

Prepare the eggs:
Peel the cooled eggs, then slice them in half lengthwise. Carefully remove the yolks and place them in a bowl. Add half of the tuna mixture to the yolks and mash everything together with a fork until smooth. You can adjust the consistency by adding more mayo if needed.

Fill the eggs:
Spoon or pipe the tuna mixture into the hollowed-out egg whites. You can use a piping bag with a decorative tip for a neater presentation or simply spoon it in.

Garnish and serve:
Sprinkle the filled deviled eggs with paprika or smoked paprika for color and extra flavor. Garnish with chopped fresh parsley or dill for a fresh, herby touch. Serve immediately or refrigerate until ready to serve.