Mash the chick peas with the onion in a large bowl until mostly smooth. I use a potato masher, but you can also pulse it in a blender of food processor. Mash in the carrots and celery, then mash in the mayo, vinegar and salt. Taste and add a drop of two more of vinegar, or another pinch of salt if it doesn't taste like much yet. Spoon into buns or pitas and garnish with tomatoes and lettuce.