01 - Preheat oven to 375°F and grease a 9x13-inch baking dish with cooking spray or butter.
02 - Bring a large pot of salted water to a boil. Cook pasta until al dente according to package directions. Drain well and set aside.
03 - In a large skillet over medium heat, melt 3 tbsp 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 remove the raw flour taste.
05 - Gradually whisk in chicken broth and whole milk, stirring to incorporate and prevent lumps. 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, black pepper, and salt. 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 well combined with the creamy chicken mixture.
08 - Pour the pasta 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 casserole. Bake for 20–25 minutes until sauce is bubbling around edges and topping is golden brown.
11 - Remove from oven and let rest for 5 minutes to allow the sauce to set slightly for easier serving.