1.Preheat oven to 425 degrees. Spray mini cupcake pans with cooking spray.
2.Combine chicken, carrots, peas and celery in saucepan. Add just enough water to cover ingredients and bring to a boil. Cook 15 minutes then remove from heat; strain and set aside.
3.Cook onions in butter on medium heat until soft and translucent. Stir in flour, salt and black pepper. Slowly stir in chicken broth and milk. Simmer over medium-low heat until very thick, stirring occasionally. Remove from heat, mix in chicken and vegetables; set aside
4.Unroll pie dough and cut 32 circles with a 3½” cookie cutter. Fit 16 pie dough circles into bottom of mini muffin pan, making sure there’s enough dough left on top to pinch together with the top.
5.Spoon chicken mixture into pie crust heaping slightly. Brush edges of bottom pie crusts with egg wash to adhere top crust. Place remaining pie tops on each cup; pinch top and bottom layers together. Flute edges or press with fork to seal.
6.Cut small slits into tops so steam can escape during baking and brush egg wash over pies for a golden, shiny finish.
7.Bake 10–15 minutes or until crust is golden brown and filling starts to bubble. Let cool at least 10 minutes before serving