01 - Preheat the oven to 400°F.
02 - In a large skillet over medium heat, melt butter. Add onion, carrots, and celery; sauté 5–7 minutes until vegetables soften. Add garlic and cook an additional minute.
03 - Sprinkle all-purpose flour evenly over vegetables and stir for 1 minute, coating the mixture.
04 - Gradually whisk in chicken broth and whole milk. Continue cooking, stirring often, until mixture thickens, approximately 4–5 minutes.
05 - Stir in cooked chicken, frozen peas, thyme, salt, and black pepper. Remove pan from heat and let the filling cool briefly.
06 - Spoon the filling into a 9-inch pie dish or divide among four individual oven-safe ramekins.
07 - On a lightly floured work surface, roll out the pie dough to cover your dish. Using a sharp paring knife, carve facial features into dough for a jack-o'-lantern effect.
08 - Place the carved dough over the filling, pressing edges to seal. Brush dough with beaten egg for enhanced color.
09 - Bake 25–30 minutes (or 18–22 minutes for single portions), until crust is golden and filling is bubbling.
10 - Allow to cool 10 minutes before serving for best texture.