preheat oven to 375°F

in pot add butter to flour , whisk until smooth. cook over med-high heat 6-8 minutes or until mixture is light golden brown, whisk occasionally. add stock, whisk constantly until smooth. add thyme, pressed garlic, salt and black pepper. simmer, uncovered, 6-8 minutes or until sauce thickens. remove from heat, add chicken and milk.

meanwhile, finely chopped onion. add onion, frozen vegetables and chicken mixture to deep dish, mix well. unfold pastry onto lightly floured surface; seal seems and roll to an 11 1/2 inch length. using pastry cutter fitted with fluted wheel, score pastry lengthwise in half then crosswise into four rectangles to make eight rectangles. (do NOT cut all the way through pastry) lay pastry over filling; press edges against sides of dish to seal. trim excess pastry. in a desperate bowl, whisk together egg and water; lightly brush over pastry.

bake, uncovered, 35-40 minutes or until pastry is golden brown. remove from oven, let stand 10 minutes before serving.