Peel hard boiled eggs and mash them up in a large bowl with the Greek yogurt. Season with salt and pepper.
In a separate medium bowl, mash avocado.
Add mashed avocado to bowl with mashed egg and stir together with chopped celery and spinach.
Stir in salt and hot pepper flakes (if using).
Make sandwiches with avocado egg salad using nice crusty bread, toasted if desired.