In a mixing bowl, combine drained tuna, mayonnaise, Dijon mustard, chopped celery, diced red onion, diced red bell pepper, and fresh parsley. Mix well until all ingredients are evenly incorporated.

Season the tuna mixture with salt and pepper according to your taste preferences. Remember, a pinch of salt can elevate the flavors without overpowering.

Toast the slices of whole grain bread to your desired level of crispiness.

Evenly distribute the tuna salad mixture onto two slices of the toasted bread.

Top each with a few lettuce leaves for a refreshing crunch.

Complete the sandwich with the remaining slices of toasted bread, pressing gently to ensure the ingredients meld together.

Slice the sandwich diagonally for a visually appealing presentation.