1-Start: Start by preheating your oven to 425°F (220°C) and getting all ingredients ready for smooth cooking. First, season and simmer the chicken breasts until fully cooked, then chop them into bite-sized pieces to prepare the base of your filling.

2-Next: Next, sauté the vegetables like celery and onion in butter over medium heat until they soften and release their flavors.

3-After that: After that, stir in the flour and seasonings such as salt, pepper, celery seed, and garlic powder to form a roux that will thicken your sauce.

4-Gradually: Gradually add the reserved chicken cooking water and milk, whisking constantly to create a smooth, creamy mixture.

5-Once thickened: Once thickened, mix in the chopped chicken and frozen mixed vegetables, then let the filling cool slightly before assembling.

6-Roll out: Roll out the pie crust dough to line a 9-inch pie pan, add the cooled filling, and top with the second crust. Seal and crimp the edges, make slits for steam to escape, and brush the top with a mixture of egg and milk for a golden finish.

7-Bake: Bake for 40-50 minutes until the crust is golden brown and the filling bubbles, using foil if the crust browns too fast, and let it rest for 15-20 minutes before serving for the best texture and flavor.