Set oven to 400°F.Grease the inside of a 9x9” casserole dish. On a lightly floured surface, roll the pastry sheet into a 12-inch square. Gently press the pastry dough in and up sides of dish. Trim the excess pastry. Using a fork, prick the pastry thoroughly. Place a sheet of aluminum foil against the surface of the pastry and cover sides so they don’t over cook and burn. Bake for 20 minutes. Remove dish from oven and remove the aluminum foil, then place an empty baking sheet into the oven to preheat.
Heat butter in a large skillet over medium heat, then add onions, celery, carrots and potatoes. Sauté for 5 minutes, or until the vegetables are tender-crisp and slightly browned. Sprinkle flour over vegetables, stir and cook for another 4 minutes. Slowly whisk in the stock and wine, cooking until the mixture bubbles and thickens.
Stir in mustard, thyme, sage, white pepper, turkey, peas and parsley.
Spoon the chicken vegetable mixture into the the baked bottom crust. Top with remaining puff pastry sheet, or cut into strips to create lattice crust (See Note 2) Place the turkey casserole onto a baking sheet and bake for 30-35 minutes or until the filling is hot and bubbling and the pastry is golden brown.
If substitute is needed for wine, use chicken stock.
On a lightly floured surface, cut the puff pastry sheet crosswise into 4 strips and halve each one, for 8 strips total. Weave a lattice pattern over the filling with the pastry strips. Trim any excess pastry. Brush top of pastry with egg wash.