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 tablespoons 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 coat the vegetables and remove any raw flour taste.
05 - Gradually whisk in chicken broth, followed by whole milk. Bring the mixture to a gentle simmer, whisking frequently. 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, carrots, and corn. Simmer for 2 minutes until everything is heated through.
07 - Add the cooked pasta to the skillet. Gently fold until pasta is evenly coated with the creamy chicken and vegetable mixture.
08 - Pour the entire 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 crumbs are evenly coated.
10 - Sprinkle the breadcrumb mixture evenly over the casserole. Bake for 20–25 minutes until the sauce is bubbling around the edges and the topping is golden brown.
11 - Remove from oven and let the casserole rest for 5 minutes before serving to allow the sauce to set slightly for easier scooping.