Rinsing or soaking red onion can remove some of the sharp bite of the allium and reduce the chances you will be tasting it all day. In this vegetarian sandwich, it also keeps it from dominating all the other ranch-y flavors and fresh herbs.
Rinse the red onion under cold water; transfer to a small bowl. Add 1 tablespoon lemon juice and let stand 5 to 10 minutes.
Meanwhile, combine the chickpeas, ranch dressing, a pinch of salt and a generous grinding of pepper in a large bowl. Smash together with a potato masher or sturdy fork until the chickpeas are in small pieces. Add the red onion, celery, parsley, dill, 1 teaspoon lemon juice and salt and pepper to taste; stir to combine.
Lay out 4 slices of bread and top each with 1 or 2 lettuce leaves. Spread the chickpea salad (about 1/2 cup per sandwich) in an even layer on top. Season the avocado slices lightly with salt and pepper and drizzle with the remaining 1 teaspoon lemon juice. Divide the avocado among the sandwiches and top each with a second piece of bread. Serve the sandwiches with chips and pickles.