In a medium-sized mixing bowl, combine two cans of thoroughly drained tuna with approximately ¼ cup of mayonnaise. Add in finely chopped celery, red onion, sweet pickle relish, and Dijon mustard. Mix all the ingredients together with a fork until they are creamy and well-combined, taking about 2-3 minutes.

After mixing the tuna salad, season it with salt and pepper according to your preference. Give the mixture a quick taste test for seasoning balance; adjust as needed.

If desired, lightly toast two slices of your favorite bread in a toaster or on a skillet over medium heat for about 2-3 minutes until golden brown.

Spread the creamy tuna salad evenly over one slice of the chosen bread. If you’d like to include fresh ingredients, layer on crispy lettuce and slices of juicy tomato on top of the tuna mixture.

Top the sandwich with the second slice of bread, pressing down gently to secure the layers. Cut the sandwich diagonally if desired and serve immediately.