Add the chickpeas to a medium bowl. Mash them with a potato masher or the back of a fork until mostly broken down, leaving a few chickpeas whole for texture.
Add the avocado and mash it into the chickpeas until combined.
Add the lemon juice, minced garlic, green onions, celery, dill, chopped pickles, salt, and black pepper.
Stir everything together until the mixture is creamy and well combined. Taste and adjust the seasoning, adding more lemon juice or salt if desired.
To assemble the chickpea avocado sandwich, spread the smashed chickpea mixture over a slice of bread.
Add lettuce or arugula and a little mustard, if desired. Top with another slice of bread.
Slice and serve immediately.