Begin by melting butter over medium heat in your saucepan. Once it’s foaming, add your diced onion and cook until it starts to soften, about 2 minutes. Stir in minced garlic and cook for another 30 seconds—watch carefully so it doesn’t burn. This aromatic base forms the backbone of your filling.
Next, whisk in your flour. Keep whisking for about 30 seconds to form a light roux; this removes the raw flour taste. Slowly pour in the chicken broth and milk, whisking constantly until the mixture thickens and begins to bubble, which takes around 2 minutes.
Remove from heat and stir in garlic powder, onion powder, salt, pepper, and thyme. Then fold in the frozen peas and carrots along with the shredded chicken, stirring until well coated. This is where the dish comes together visually and flavorfully.
Preheat your oven to 400 °F. Transfer the savory filling into your casserole dish. Roll out your puff pastry just enough to cover the dish with a half‑inch overhang; keep it chilled until assembly to preserve puff.
Drape the pastry gently over the filling. Use a sharp knife to cut 3–4 slits on top—this vents steam so the layers puff beautifully and crust stays crisp.
In a small bowl, whisk together the egg and water. Brush a thin layer over the pastry’s surface—this is what gives it that irresistible golden sheen.
Place your dish on the middle rack and bake for 25–30 minutes, until the pastry is golden brown and the filling bubbles up through the vents. If the pastry browns too quickly, loosely tent with foil after about 15 minutes.
Once baked, let the pie rest for about 5 minutes—this helps set the sauce so it’s easier to slice. Sprinkle minced fresh parsley on top for a fresh finish and serve right from the dish, letting everyone scoop out their own comforting portion.