Roll each pastry sheet into a 9 x 13 rectangle on a floured board or parchment. Cut one sheet in to 9 horizontal 1 inch wide strips. Cut the other sheet in to 13 vertical 1 inch wide strips. (I used a pizza cutter to cut the strips.) On a parchment or Silpat line baking sheet, weave strips into a 9 x 13 lattice pattern.

Put the crust on the baking sheet in the freezer while preparing the filling.

Preheat oven to 425º. Spray a 9x13 pan with nonstick cooking spray.

In large saucepan, heat butter over medium heat until melted. Add onion and cook until tender, 3 - 5 minutes. Whisk in flour, salt and pepper. Cook, stirring constantly, for 2 to 3 minutes until smooth and bubbly.

Slowly add chicken broth and half and half, stirring constantly until the sauce is thick and smooth, about 2 minutes. Add more or less half and half as needed.

Stir in chicken, potatoes, carrots and frozen peas. Pour into prepared casserole dish.

Remove puff pastry crust from the freezer. Put the baking sheet in the oven on the top rack and the casserole on the bottom rack and bake uncovered 25 to 30 minutes until the crust is golden brown and filling is bubbly.

Put puff pastry crust on top of pie right before serving.