01 -
Preheat the oven to 375°F (190°C) and grease a 9x13-inch baking dish.
02 -
Boil penne pasta until al dente, then drain and set aside.
03 -
Melt butter in a large skillet over medium heat. Add diced onion, carrots, and celery; cook for 5-7 minutes until softened. Stir in minced garlic and cook for an additional minute.
04 -
Sprinkle flour over the cooked vegetables and stir continuously for 1-2 minutes to form a roux.
05 -
Gradually whisk in chicken broth and whole milk, stirring until the mixture becomes smooth. Simmer until the sauce thickens enough to coat the back of a spoon.
06 -
Add cooked chicken, frozen peas, dried thyme, salt, and pepper to the sauce. Stir well and let simmer for a few minutes.
07 -
Incorporate the drained pasta into the sauce mixture and combine gently.
08 -
Transfer the pasta mixture into the prepared baking dish. Lay the thawed puff pastry sheet over the top, trimming edges if necessary and tucking slightly.
09 -
Brush the surface of the puff pastry with the beaten egg to ensure a golden finish.
10 -
Bake for 25-30 minutes until the puff pastry is golden brown and the filling is bubbling.
11 -
Allow the dish to rest for 5-10 minutes before serving to set.