1. Grate the potato by hand on a large shred and place in a mixing bowl.
2. Place the onions, celery, and pickles in a food processor fitted with a blade attachment. Pulse mixture just a few times until the vegetables are chopped but not too fine. Add to the grated potatoes.
3. Place the tuna, mayonnaise, and black pepper into the processor and blend just until smooth. Add to the bowl of vegetables and mix to combine.