01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Bring a large pot of salted water to a boil. Add pasta and cook until al dente according to package directions. Drain well and set aside.
03 - In a large skillet, melt 3 tbsp butter 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 sautéed vegetables. Cook for 1 minute, stirring constantly to prevent burning, until the flour mixture smells nutty.
05 - Gradually whisk in chicken broth and whole milk, ensuring no lumps form. Bring the mixture to a simmer and cook for 3-4 minutes, stirring frequently, until the sauce thickens enough to coat the back of a spoon.
06 - Stir in dried thyme, parsley, black pepper, and salt. Add shredded chicken, frozen peas and carrots, and frozen corn. Cook for 2 minutes until everything is heated through.
07 - Add the cooked pasta to the skillet. Mix gently but thoroughly until all ingredients are well coated with the creamy sauce.
08 - Pour the entire chicken and 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 the breadcrumbs are evenly coated with butter.
10 - Sprinkle the breadcrumb and Parmesan mixture evenly over the top of 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 - Remove from oven and let the casserole rest for 5 minutes before serving to allow the sauce to set slightly.