Place the chickpeas in a medium mixing bowl. Mash them with a potato masher or the back of a fork until mostly broken down, leaving a few larger pieces for texture.
Add the ripe avocado and mash it into the chickpeas until creamy but still slightly chunky.
Add the lemon juice, minced garlic, diced celery, chopped dill pickles, fresh dill, and green onions.
Season with sea salt and freshly ground black pepper. Stir everything together until evenly combined.
Taste the Smashed Chickpea Avocado Sandwich filling and adjust with additional lemon juice, salt, pepper, dill, or pickles if desired.
Spread mustard over the bread if using. Divide the avocado chickpea mixture between slices and add lettuce or arugula.
Top with the remaining bread slices and serve immediately. You can also spoon the filling into pitas or wrap it in tortillas.