01 - Preheat oven to 375°F and grease a 9x13-inch baking dish.
02 - Cook pasta in salted boiling water until al dente, then drain and set aside.
03 - Melt 3 tbsp butter in a large skillet over medium heat. Add onion, celery, and garlic; sauté 3–4 minutes until softened.
04 - Sprinkle flour into the skillet and cook for 1 minute, stirring constantly to form a roux.
05 - Gradually whisk in chicken broth and milk. Bring to a simmer and cook 3–4 minutes until thickened.
06 - Stir in thyme, parsley, salt, and pepper. Add shredded chicken, peas, carrots, and corn; cook for 2 minutes until heated through.
07 - Add cooked pasta to the skillet and mix until well combined with the sauce and vegetables.
08 - Pour the mixture into the prepared baking dish and spread evenly.
09 - In a small bowl, combine panko, Parmesan, and melted butter. Sprinkle evenly over the casserole.
10 - Bake for 20–25 minutes until the casserole is bubbling and the topping is golden brown.
11 - Let the dish cool for 5 minutes before serving to allow the sauce to set slightly.