Combine all of the ingredients for the yogurt sauce together in a small bowl and whisk until combined. Set aside.
Spiralize the apple using the ribbon blade and sprinkle with lemon juice to prevent it from turning brown and set aside.
In a medium bowl mix together tuna, celery, and yogurt sauce.
Slice off the tops of the peppers, cut them in half and remove the seeds.
Fill each bell pepper with tuna mixture. Top with arugula and spiralized apples.