01 - Set oven to 400°F and allow to preheat before beginning further steps.
02 - Melt unsalted butter in a large skillet over medium heat. Add diced onion, minced garlic, carrots, and celery. Sauté for 5 minutes until vegetables soften.
03 - Incorporate the all-purpose flour into the vegetable mixture, stirring continuously for 1 to 2 minutes to cook out the raw flavor.
04 - Gradually pour in chicken broth and milk, stirring constantly to prevent lumps. Cook for 3 to 4 minutes until the mixture shows noticeable thickening.
05 - Integrate shredded cooked chicken, frozen peas, salt, ground black pepper, dried thyme, and dried sage into the skillet. Simmer gently for 2 to 3 minutes. Remove pan from heat once fully combined.
06 - Transfer the hot filling evenly into a 9-inch round pie dish.
07 - On a lightly floured surface, roll out puff pastry until large enough to fit your pie dish. Use a sharp knife or pizza cutter to slice pastry into 1-inch wide strips.
08 - Arrange pastry strips across the filling in a crisscross fashion, leaving small gaps to simulate bandages.
09 - Position two black olive slices on one end for 'mummy eyes.' Brush pastry with beaten egg for shine.
10 - Place the pie in the oven and bake for 35 to 40 minutes until pastry is golden and crisp. Allow to cool for 10 minutes before serving.