01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Boil pasta in salted water until al dente according to package directions. Drain thoroughly and set aside.
03 - Melt 3 tbsp butter in a large skillet over medium heat. Add diced onion, celery, and minced garlic. Sauté for 3–4 minutes until vegetables are softened and fragrant.
04 - Sprinkle flour evenly over the vegetables. Cook for 1 minute, stirring constantly to remove raw flour taste and create a smooth paste.
05 - Gradually whisk in chicken broth followed by whole milk, ensuring no lumps form. Bring mixture to a gentle simmer and cook for 3–4 minutes until sauce thickens enough to coat the back of a spoon.
06 - Stir in dried thyme, parsley, salt, and black pepper. Add shredded chicken, frozen peas and carrots, and corn. Cook for 2 minutes until everything is heated through.
07 - Add cooked pasta to the skillet and fold gently until evenly distributed throughout the creamy chicken mixture.
08 - Pour the pasta mixture into the prepared 9x13-inch baking dish, spreading it into an even layer.
09 - In a small bowl, combine panko breadcrumbs, grated Parmesan cheese, and melted butter. Mix until crumbs are evenly coated.
10 - Sprinkle the breadcrumb mixture evenly over the entire surface of the casserole.
11 - Bake for 20–25 minutes until sauce is bubbling around the edges and topping is golden brown and crispy.
12 - Let the casserole rest for 5 minutes before serving to allow the sauce to set slightly for easier scooping.