In a big bowl, mix celery, red onion, and grated chicken.
Include lemon juice, mayonnaise, and Dijon mustard.
Mix till it properly binds.
Sprinkle and mix black ground pepper and salt as you like.
Make horizontal slices of keto croissants.
Spread a filling portion of chicken salad over the bottom half of every croissant.
Include optional toppings like lettuce or avocado if you want.
Lay the other side of the croissant over the filling, pressing it with gentle hands.
Do quick serving, or cool it for later serving.