Wash the cucumbers thoroughly under cool running water. Slice them in half lengthwise and scoop out the seeds gently with a spoon to create a boat shape.

In a medium mixing bowl, combine the canned tuna, mayonnaise, mustard, and diced celery. Mix until well-blended and creamy. Adjust seasoning with salt and pepper.

Spoon the tuna mixture into each cucumber boat carefully, filling them generously but avoiding overfilling.

Serve immediately, garnished with freshly chopped herbs if desired.