01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with cooking spray or butter.
02 - Cook pasta in salted boiling water until al dente according to package directions. Drain well and set aside.
03 - In a large skillet, melt 3 tbsp butter over medium heat. Add diced onion, celery, and minced garlic; sauté for 3–4 minutes until 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, smoothing out any lumps. Bring 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, carrots, and corn. Cook for 2 minutes until everything is heated through.
07 - Add cooked pasta to the skillet and mix until well combined with the creamy chicken mixture.
08 - Pour the pasta mixture into the prepared 9x13-inch baking dish, spreading evenly.
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 - Remove from oven and let cool for 5 minutes before serving to allow the sauce to set slightly for easier scooping.