Rinse the chickpeas and add to a food processor.
Add in the dijon mustard, red onion, celery, and relish (everything except the mayo and salt and pepper). Pulse until a flaky consistency has formed. You don't want the mixture to be completely smooth. Keep some of the texture of the chickpeas but you shouldn't have full chickpeas in the mixture.
Transfer the mixture to a separate bowl and stir in the mayo and season it with salt and pepper.
There is enough chickpea salad for roughly 4 sandwiches. Divide evenly among the sandwiches. Top with extra veggies (romaine, cucumber). If you have extras, store the rest in a sealed container for 2-3 days.