Add chickpeas in a medium bowl and use a potato masher or the back of a ladle to smash about half of them. Having some whole adds more texture.
Add celery, parsley, Greek yogurt (or mayonnaise), mustard, and a splash of lemon juice, and combine—season with salt and pepper to taste.
Add the optional crushed red pepper flakes and desired amount of sliced jalapeños to taste, and mix to combine.
To serve, add a generous spoonful of the chickpea salad to a slice of toasted bread, top with sliced tomatoes, green onions, and arugula. Top with another piece of bread.
Store any leftovers in the refrigerator for a couple of days.