Follow pie crust instructions for letting crust rest before using. Preheat oven to 400°F.
Heat butter over medium heat in large pot until melted. Add onion and cook, stirring occasionally, 5 minutes, or until translucent. Add kale and cook 3 minutes, or until wilted. Sprinkle vegetables with flour and stir. Slowly add milk while stirring. Bring to a boil, reduce heat to a simmer and add chicken, peas, mustard, thyme, garlic powder and salt. Stir occasionally until heated through.
Meanwhile, unroll pie crust and cut into 3-inch rounds.
Spray baking sheet with nonstick cooking spray and place rounds on pan. Whisk egg and water in small bowl and brush top of rounds.
Bake pie rounds 9 minutes, or until golden brown. Brush again with egg mixture. Let cool.
Ladle chicken stew into thermos. Wrap pastry circles in plastic wrap to be packed in lunchbox alongside stew and placed on top before eating.