In a medium size bowl roughly mash the chickpeas. Add in the onion, celery, and relish and mix with a spoon. Then mix in the mayo, Dijon mustard, garlic powder, salt, and pepper. Taste test for seasoning adjustments.
Place romaine lettuce, or greens of choice, on two slices of bread and scoop an even amount of filling on top of each one. Top with another leaf of romaine lettuce and other two slices of bread. Filling lasts up to 3 days in the fridge.