Drain and rinse one can of chickpeas, then transfer them to a mixing bowl. Mash the chickpeas until slightly chunky.

Add chopped celery, lemon juice, mayonnaise, Dijon mustard, and diced onion to the mashed chickpeas. Stir until well combined.

Spread the chickpea salad mixture onto one side of each bread slice. If using cheese, sprinkle it on some slices.

Heat a skillet over medium heat with a little butter or oil. Grill the sandwiches for about 5-7 minutes until the bottom is golden brown.

Flip the sandwiches carefully and grill the other side for an additional 5-7 minutes until golden and cheese is melted.

Remove sandwiches from the skillet, let cool for a minute, then cut in half and serve warm.