Chickpea Salad Sandwich

Name: Chickpea Salad Sandwich
Ingredients: 1 (15.5-oz.) can chickpeas (garbanzo beans), drained and rinsed; 1 celery stalk, chopped (about ½ cup); ½ cup loosely packed fresh flat-leaf parsley, roughly chopped; 2 tablespoons mayonnaise (preferably made with avocado oil or olive oil), plus more for serving; 1 small (about 1 oz.) shallot, finely chopped (about 2 Tbsp.); 1 ½ tablespoons fresh lemon juice (from 1 lemon); ½ teaspoons Dijon mustard, plus more for serving; ½ teaspoon fine sea salt; Freshly ground black pepper; 4 whole-wheat, seven-grain or sourdough bread slices, toasted; 2 red leaf lettuce leaves