Heat olive oil in a large skillet, add chicken and cook until done. Remove chicken from skillet.
Add celery & onion to skillet, cook until soft. Mix in the frozen vegetables, chicken soup, half & half, salt, pepper, garlic powder and soy sauce. Cover and cook until all the vegetables are done. Add in the chicken.
Pour the chicken/vegetable mixture into a baking dish.
Preheat oven to 400 degrees.
Whisk together egg and 1 tbsp water in a small bowl.
Roll pastry sheet into a 12 inch square on a lightly floured surface. Cut pastry sheet into 3 inch squares. Arrange pastry squares on top of chicken/vegetable mixture in baking dish, brushing each square with egg mixture and slightly overlapping each square, covering the entire top of the casserole.
Bake until top is browned, about 30 minutes. Let stand about 10 minutes prior to serving.