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. Cook pasta according to package directions until al dente, then drain 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 sautéed vegetables. Cook for 1 minute, stirring constantly to coat and remove raw flour taste.
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 until sauce thickens enough to coat the back of a spoon.
06 - Stir in dried thyme, dried parsley, black pepper, and salt until well incorporated.
07 - Add shredded chicken, frozen peas and carrots, and frozen corn to the sauce. Cook for 2 minutes until everything is heated through. Add cooked pasta and toss until evenly coated.
08 - Pour the 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 breadcrumbs are evenly coated.
10 - Sprinkle the breadcrumb mixture evenly over the top of the casserole.
11 - Bake for 20–25 minutes until the filling is bubbling around the edges and the topping is golden brown and crispy.
12 - Remove from oven and let rest for 5 minutes to allow the casserole to set slightly before serving.