Preheat the oven to 400 degrees.
In a large cast iron skillet, melt butter over medium heat. Add onion, carrot, and celery. Cook until the onion is softened, about 3-4 minutes. Add garlic and thyme and cook for 1 minute more.
Sprinkle flour over the veggies and stir to coat. Cook, stirring, for 1 minute to eliminate raw flour taste.
Slowly pour in chicken broth, stirring constantly to avoid lumps.
Once smooth, add the milk or cream. Bring to a gentle simmer and cook until just starting to thicken, about 3 minutes.
Turn off the heat and stir in shredded rotisserie chicken and frozen peas. Season generously with salt and pepper. Taste and adjust seasoning as needed.
On a lightly floured surface, gently roll out the puff pastry and cut into 16-24 equal squares. Arrange as you like on top of the chicken mixture, overlapping slightly.
Brush the top with egg wash. Place the skillet on a parchment paper-lined baking sheet to catch any drips. Bake for 25–30 minutes, or until the puff pastry is deep golden brown and crisp.
Let rest for 5–10 minutes before serving. Serve directly from the skillet and enjoy!
