Cut the avocados in half and remove the pit. Scoop out a bit of the flesh to create a larger cavity for the filling if desired. Reserve the scooped avocado for another use (like guacamole or smoothies).

Make the tuna salad:

In a mixing bowl, combine the drained tuna, mayonnaise, Dijon mustard (if using), diced celery, diced red onion, lemon juice, salt, and pepper. Mix well until all ingredients are combined.

Fill the avocado boats:

Generously spoon the tuna salad into each avocado half, packing it in slightly.

Garnish and serve:

If desired, sprinkle chopped parsley or cilantro on top for added flavor.

Serve the avocado and tuna salad boats on a bed of lettuce leaves for a nice presentation, if using.