Preheat your oven to 400°F (205°C). Season the cubed chicken breast generously with salt, pepper, and garlic powder on all sides.

Melt 1 tablespoon of butter in a large, deep skillet or Dutch oven over medium-high heat. Add the chicken in a single layer and cook for 4 to 5 minutes, turning once, until lightly golden and just cooked through. Transfer to a plate and set aside. You do not need the chicken fully browned — it will finish cooking in the oven.

In the same skillet, melt the remaining 3 tablespoons of butter over medium heat. Add the diced onion, celery, and carrots. Season lightly with salt and cook, stirring occasionally, for 6 to 8 minutes until the vegetables have softened and the onion is translucent.

Add the minced garlic and fresh thyme. Cook for 1 minute more, stirring constantly, until fragrant.

Sprinkle the flour over the softened vegetables and stir to coat everything evenly. Cook the flour for 1 to 2 minutes, stirring constantly, to remove the raw flour taste and build the roux.

Slowly pour in the warm chicken broth, about half a cup at a time, whisking constantly to prevent lumps. Once all the broth is incorporated, pour in the whole milk and whisk until smooth.

Bring the mixture to a gentle simmer over medium heat and cook, stirring frequently, for 3 to 4 minutes until the sauce has thickened to a creamy, gravy-like consistency. Taste and adjust seasoning with salt and pepper.

Remove the skillet from heat. Stir in the cooked chicken pieces and thawed peas until evenly distributed. The filling should be thick, creamy, and well-seasoned. Allow it to cool slightly for 5 minutes.

Unroll one pie crust and gently press it into a 9-inch deep-dish pie plate, allowing the edges to hang slightly over the rim.

Pour the warm filling into the prepared crust and spread into an even layer.

Unroll the second pie crust and lay it over the top of the filling. Trim any excess dough, then press the edges of the two crusts together and crimp decoratively with a fork or your fingers to seal.

Use a sharp knife to cut 4 to 5 slits or a small shape in the center of the top crust to allow steam to escape.

Brush the entire surface of the top crust generously with the egg wash. Sprinkle with a pinch of flaky sea salt if desired.

Place the pie on a foil-lined baking sheet to catch any drips. Bake at 400°F for 35 to 45 minutes, until the crust is deep golden brown and the filling is visibly bubbling through the vents.

If the edges of the crust begin to brown too quickly before the center is done, loosely tent the edges with aluminum foil and continue baking.

Remove from the oven and allow the pie to rest for at least 10 to 15 minutes before slicing and serving. This resting time allows the filling to set so slices hold their shape beautifully.