I used to work at a beachside restaurant in Mallorca called Patiki and we served something very similar to this and everyone went mad for it (I think it’s still on the menu). It’s a reminder of how great it is to have leftover chicken in the refrigerator.
Personally, I love celery and I think it’s a key part of this recipe, providing a delicious crunch, but feel free to replace it with cucumber if it’s not your thing. I’d recommend using a flavoured aioli for this, or even adding a tablespoon of curry powder and a few raisins to mayonnaise to make it more of a coronation chicken bun. It’s also great as a simple salad if you’d rather not have the bun. Shred up a load of lettuce, then toss with the chicken mixture.