Preheat oven to 400°F. Lightly coat a 2- to 3-quart baking dish with cooking spray. Spread mashed potatoes in an even layer in bottom of prepared baking dish.
Whisk together gravy, mustard, thyme, pepper, and 3/4 cup of the heavy cream in a large bowl. Taste and add salt as needed. Stir in turkey and vegetables until well coated. Transfer mixture to baking dish, spreading evenly over potatoes. Spoon cranberry sauce over top.
Cover dish tightly with aluminum foil. Place on a rimmed baking sheet, and bake in preheated oven until center is warm to touch, about 35 minutes.
During last 10 minutes of baking, roll puff pastry on a lightly floured surface until pastry is 2 inches larger than length and width of baking dish. Whisk egg and remaining 2 teaspoons heavy cream in a small bowl until combined; set aside.
Remove baking dish from oven; remove and discard foil, and place pastry on top of cranberry sauce. Trim excess pastry, if needed, leaving a 1-inch overhang on all sides. Fold overhang underneath itself, and crimp edges as desired. Brush pastry with egg mixture, and cut 3 or 4 small slits in pastry for steam to escape. Return to oven, and bake at 400°F until pastry is puffed and brown, filling is bubbling, and a thermometer inserted in center registers 165°F, 20 to 25 minutes.
Let cool 15 minutes before serving. Garnish with additional thyme.