01 -
Preheat the oven to 375°F (190°C). Grease a 9x13-inch baking dish.
02 -
Boil penne pasta until al dente. Drain thoroughly and set aside.
03 -
Melt butter in a large skillet over medium heat. Add diced onion, carrots, and celery; sauté for 5 to 7 minutes until softened. Add garlic and cook for an additional minute.
04 -
Sprinkle flour over the vegetables and stir well. Cook the mixture for 1 to 2 minutes to eliminate raw flour taste.
05 -
Gradually whisk in chicken broth and whole milk, stirring constantly until smooth. Simmer until the sauce thickens enough to coat the back of a spoon.
06 -
Stir in cooked chicken, frozen peas, dried thyme, salt, and pepper. Allow the mixture to bubble gently for a few minutes to meld flavors.
07 -
Add the cooked pasta to the skillet and gently fold until evenly coated with the sauce.
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 in slightly.
09 -
Brush the surface of the puff pastry with the beaten egg to promote a golden finish.
10 -
Place in the oven and bake for 25 to 30 minutes, or until the puff pastry is golden brown and the filling is bubbling.
11 -
Allow the dish to cool for 5 to 10 minutes before serving to set the filling.