Rinse and drain a can of chickpeas. Mash the chickpeas in a medium bowl until mostly smooth, leaving some small chunks for texture.

Add mayonnaise, diced celery, Dijon mustard, and finely chopped onion to the mashed chickpeas. Mix until well combined. Season with salt and pepper.

Spread the chickpea mixture between two slices of bread. If using cheese, sprinkle it on top before adding the second slice.

Heat a non-stick skillet over medium heat, add butter, and grill each sandwich for 2-4 minutes per side until golden brown.

Remove sandwiches from skillet, let sit for a minute, slice in half, and serve warm.