Put drained chickpeas in a medium bowl. Using a stick blender (or a fork), mash the chickpeas. You want a chunky mix, with most chickpeas completely mashed and some only broken up.
Add lemon juice, mayonnaise, mustard, chopped celery, pinch of salt and pepper. Mix thoroughly. Taste and adjust seasoning to your taste. Add more lemon juice and mayonnaise too, if needed, until you get a desired consistency.
Spread a hefty amount of the chickpea salad over one slice of bread (toast it if you prefer that). Top with sliced tomatoes and 1-2 big leaves of lettuce. Add other toppings if desired, cover with a second slice of bread. Repeat for more sandwiches and serve.
Keep leftover chickpea salad covered in the refrigerator and use within a few days.