Thaw the pastry sheet at room temperature for 40 minutes or until it's easy to handle. Heat the oven to 400°F. Stir the egg and water in a small bowl with a fork.

Stir the chicken and vegetables in a 13 x 9 x 2-inch baking dish.

Heat the butter in a 2-quart saucepan over medium heat. Add the flour and cook for 3 minutes, stirring constantly. Gradually stir the stock into the saucepan and heat to a boil. Reduce the heat to low. Cook and stir for 3 minutes or until the mixture boils and thickens. Stir in the black pepper. Pour the stock mixture over the chicken mixture.

Unfold the pastry sheet on a lightly floured surface. Roll the pastry sheet into a 13 x 9-inch rectangle. Place the pastry over the filling. Press the pastry sheet to the edges of the dish to seal. Flute the edges of the pastry, if desired. Brush the pastry with the egg mixture and sprinkle with the herbs. Cut several 2-inch long slits in the pastry top.

Bake for 25 minutes or until pastry is golden and the filling is hot and bubbling.