01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Cook pasta in salted boiling water until al dente according to package directions. Drain and set aside.
03 - In a large skillet, melt 3 tbsp butter over medium heat. Add onion, celery, and garlic; sauté for 3–4 minutes until softened and fragrant.
04 - Sprinkle flour into the skillet and cook for 1 minute, stirring constantly to incorporate and cook out raw flour taste.
05 - Gradually whisk in chicken broth and milk, ensuring no lumps form. Bring to a simmer and cook for 3–4 minutes until sauce thickens enough to coat the back of a spoon.
06 - Stir in thyme, parsley, salt, and pepper. Add shredded chicken, peas, carrots, and corn; cook for 2 minutes until everything is heated through.
07 - Add cooked pasta to the skillet and toss until well combined with the creamy chicken mixture.
08 - Pour the mixture into the prepared baking dish, spreading it evenly.
09 - In a small bowl, combine panko breadcrumbs, Parmesan cheese, and melted butter. Mix until crumbs are evenly coated.
10 - Sprinkle the breadcrumb mixture evenly over the casserole.
11 - Bake for 20–25 minutes until the sauce is bubbling around the edges and the topping is golden brown and crispy.
12 - Let the casserole cool for 5 minutes before serving to allow the sauce to set slightly for easier serving.