In a mixing bowl, combine drained tuna, diced red onion, chopped celery, Dijon mustard, lime juice, salt, and pepper. Mix well.

Cut each avocado in half and remove the pits. Scoop out a small amount of flesh to create space for the filling.

Generously spoon the tuna mixture into each avocado half.

Garnish with fresh cilantro leaves and an extra squeeze of lime juice if desired.

Serve immediately or refrigerate briefly before serving.