Preheat your oven to 400°F (200°C). Unroll the thawed puff pastry sheet on parchment paper and lightly roll with a pin to smooth the seams. Cut 4 small vent slits in the center.

Melt butter in a large 10-inch or 12-inch oven-safe cast iron skillet over medium heat. Add diced onion, carrots, and celery. Sauté for 6-8 minutes until tender and fragrant.

Add minced garlic, fresh thyme, rosemary, garlic powder, salt, and black pepper. Cook for 60 seconds until aromatic.

Sprinkle the flour over the vegetables, stirring continuously for 2 minutes to cook out raw flour starch and create a fragrant roux.

Slowly pour in chicken bone broth while whisking vigorously to prevent lumps. Whisk in heavy cream. Bring to a gentle simmer for 3-4 minutes until thick, glossy, and velvety.

Stir in the shredded cooked chicken and frozen green peas. Simmer for 1 minute until warmed through. Remove skillet from heat and smooth the top evenly.

Carefully drape the prepared puff pastry sheet directly over the hot chicken filling in the skillet, gently tucking any overhang inside the rim. Brush the pastry generously with the beaten egg wash.

Bake at 400°F (200°C) for 22 to 25 minutes until the puff pastry is dramatically puffed, deep golden brown, and shatteringly crisp, and the cream filling is bubbling at the edges.

Let rest for 10 minutes to allow the sauce to settle before ladling generous portions into bowls with big shards of crisp buttery pastry.