Add the celery to a large food processor and pulse about 5 times. Add the chopped cooked chicken and pulse just a few more times until coarsely shredded.
Press the hard boiled eggs down to the bottom. Add pickle relish, mayo, and garlic powder. Pulse a few more times until everything is mixed and the texture looks right. Don’t over-process; you want a light chop, not mush. Total pulsing time should be around 10 to 15 seconds.
Taste and season with salt and pepper if needed (if your chicken is already seasoned, you might not need any).
Lightly butter the bread (if using) and toast it. Let it cool slightly.
Add lettuce and a generous scoop of chicken salad to one slice, then top with the second slice. Cut the sandwich in half and enjoy right away.
