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 boil. Cook pasta according to package directions until al dente, approximately 8-10 minutes. Drain well and set aside.
03 - In a large skillet over medium heat, melt 3 tablespoons butter. 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 eliminate raw flour taste and create a smooth roux.
05 - Gradually whisk in chicken broth and whole milk, ensuring no lumps form. Bring mixture to a gentle simmer and cook for 3-4 minutes, stirring frequently, until 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 corn. Continue cooking for 2 minutes until everything is heated through.
07 - Add cooked pasta to the skillet and toss gently until well combined with the creamy chicken mixture.
08 - Pour the entire mixture into the prepared 9x13-inch baking dish, spreading it evenly.
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 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 rest for 5 minutes before serving to allow the sauce to set slightly.