Prepare the Tuna:
Open the cans of tuna, drain the liquid, and place the tuna in a large mixing bowl.
Use a fork to flake the tuna into small pieces.
Mix the Dressing:
In a small bowl, combine the mayonnaise, Dijon mustard (if using), and lemon juice. Stir until smooth and well-combined.
Add Vegetables and Relish:
Add the diced celery, onion, and pickle relish (if using) to the tuna bowl. If you’re adding a boiled egg, chop it into small pieces and add it as well.
Combine and Season:
Pour the dressing over the tuna and vegetables. Stir everything together until evenly coated.
Season with salt and pepper to taste.
Serve:
Serve immediately as a sandwich filling, in a wrap, or on top of a bed of greens. You can also chill it in the fridge for 30 minutes before serving for a cooler, more refreshing taste.