Drain and rinse 1 can chickpeas well. Add them to a large bowl and mash with a fork or potato masher until chunky and scoopable. Stop before they turn smooth.
Add ½ cup pickles, ½ cup radishes, 1 rib celery, 1 green onion, and 2 tablespoons capers (if using). Stir in ½ cup Greek yogurt, 2 tablespoons fresh herbs, 1 tablespoon mustard, ¼ teaspoon salt and black pepper. Add 2 tablespoons mayonnaise or tahini if you want it richer and a splash of pickle juice for a brighter flavor. Mix until creamy. Taste and adjust.
Place lettuce or spinach on the bread, spoon on the chickpea salad, then add tomato and avocado if using. Close the sandwich and serve. Toast the bread first if you like.
