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. Drain well and set aside.
03 - In a large skillet over medium heat, melt 3 tbsp butter. Add onion, celery, and 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 cook out the raw flour taste.
05 - Gradually whisk in chicken broth and milk, ensuring no lumps form. Bring mixture to a simmer, then cook for 3–4 minutes until sauce thickens enough to coat the back of a spoon.
06 - Stir in thyme, parsley, salt, and pepper. Add shredded chicken, peas, carrots, and corn. Cook for 2 minutes until everything is heated through.
07 - Add cooked pasta to the skillet and fold until evenly distributed throughout the creamy chicken mixture.
08 - Pour the chicken pasta mixture into the prepared 9x13-inch baking dish, spreading 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 casserole. Bake for 20–25 minutes until sauce is bubbling around edges and topping is golden brown.
11 - Let the casserole rest for 5 minutes before serving. This allows the sauce to set slightly for easier scooping.