Step 1: Combine chickpeas, oil, lemon juice, mustard and garlic powder in a large bowl. Using a fork or potato masher, crush the chickpeas until most are mashed, but some are still whole. Stir in celery, dill, salt and pepper.
Step 2: Spread 1 tablespoon mayonnaise on 1 side of each of 4 slices of bread. Top evenly with lettuce, onion, tomato and chickpea mixture. Top with the remaining 4 slices of bread.