Combine all of the ingredients for the fayonnaise together in a small bowl and whisk until combined. Set aside.
Slice the apple halfway lengthwise and spiralize it, using Blade B. Add the apple to a large bowl with the celery, arugula, tuna and fayonnaise. Toss to combine thoroughly.
Slice the top off of both the bell peppers and slice in half. Deseed and remove any of the white inside flesh.
To assemble the sandwiches, fill each bell pepper with the apple tuna salad.
