Preheat the oven to 350 degrees.
Add the olive oil to a skillet over medium heat. Add in the celery and onions and cook for 5 minutes.
Add the frozen vegetables, rosemary, garlic, and black pepper to the pan and cook for 5 minutes.
Add the broth and the cream and simmer for 10 minutes.
Ina small bowl whisk together the water and flour until smooth.
Slowly pour the flour mixture into the pot with the vegetables and mix well.
Add the chicken and cook for 3-4 minutes.
Pour the chicken pot pie mixture into an 8 x 8 casserole dish and cover with the puff pastry. Brush with butter.
Bake for 30 minutes.
Turn on the broiler and broil until the puff pastry is golden brown.
Remove from oven and serve immediately.