How to Make It
Whisk together egg and 1 tablespoon water in a small bowl. Roll pastry sheet into a 12-inch square on a lightly floured surface. Cut into 16 (3-inch) squares. Set egg wash and pastry squares aside.
Preheat oven to 400°F with rack in lower third of oven. Melt butter in a deep 10-inch ovenproof skillet over medium-high. Add leek and carrots; cook, stirring often, until softened, about 6 minutes. Sprinkle with flour; cook, stirring constantly, 1 minute. Stir in stock; bring mixture to a simmer. Simmer, stirring constantly, until mixture thickens, 1 to 2 minutes. Stir in chicken, peas, cream, Dijon, parsley, thyme, salt, and pepper. Remove from heat.
Brush pastry squares with egg wash; arrange squares on top of hot chicken mixture in skillet, slightly overlapping so entire surface of chicken mixture is covered. Place skillet on a rimmed baking sheet.
Transfer baking sheet with skillet to preheated oven. Bake until top is browned and filling is bubbly, about 20 minutes. Remove from oven, and let stand 10 minutes. Sprinkle with fresh thyme leaves, and serve.