Cook chicken filets per specifications. Set aside to fully cool. Once cool, cut into small pieces.
Mix mayo, Greek yogurt, dill, garlic powder, salt, pepper, pickle juice, Dijon mustard, onion, and celery. Once combined, add in cubed chicken pieces, and mix.
Serve in puff pastry cups or on a croissant sandwich with crispy fried onions on top.
To make puff pastry cups, cut a sheet of puff pastry into 12 squares. Push the squares into a greased muffin tin. Bake until golden and puffy. Push down the centers with a spoon while still warm. Fill and serve.