1. Preheat the oven according to the puff pastry package instructions.
2. Bake the puff pastry shells until golden brown and flaky, then set aside.
3. In a large pan, melt butter over medium heat.
4. Add diced onions, celery, and mushrooms, then sauté until softened.
5. Sprinkle flour over the vegetables and stir to form a roux.
6. Slowly pour in chicken stock while stirring to create a smooth sauce.
7. Add cooked, diced chicken to the pan and mix well.
8. Stir in cream and season with salt, pepper, and optional herbs like thyme or tarragon.
9. Let the mixture simmer until it thickens and flavors combine.
10. If desired, add a splash of white wine or grated Parmesan for extra flavor.
11. Carefully cut the tops off the baked puff pastry shells.
12. Spoon the warm chicken filling into the pastry shells.
13. Replace the pastry lids for presentation or serve open-faced.
14. Garnish with fresh herbs, if desired, and serve immediately.