Avocado-Egg Salad Sandwiches with Pickled Celery

To prevent the avocado from browning in the leftover egg salad, place the remaining lettuce in a bowl and cover the surface with plastic wrap. Then cover the entire bowl tightly with plastic wrap.
Pour water into a large saucepan to a depth of 1 inch; place a large vegetable steamer in the pan. Bring the water to a boil over medium-high heat. Add the eggs to the steamer. Cover and steam the eggs for 16 minutes. Remove from heat. Put the eggs in a large bowl filled with ice water.While the eggs are cooking, combine 3 tablespoons of water, vinegar, and sugar in a medium-sized bowl suitable for a microwave; reheat in the microwave on high power for 2 minutes or until boiling. Add the celery; let stand for 15 minutes. Drain the water.Meanwhile, combine the avocado, mayonnaise, juice, mustard, pepper, and salt in a medium bowl, stirring well, until smooth.Peel the eggs. Cut the eggs in half lengthwise; leave 2 egg yolks for another use. Chop the remaining eggs and egg whites. Gently mix the eggs, celery, and sunflower seeds with the avocado. For 4 slices of bread, spread about 1/2 cup egg mixture, 1/4 cup arugula, 1 slice of tomato, and the remaining 4 slices of bread.