01 - Preheat oven to 375°F and grease a 9x13-inch baking dish.
02 - Cook pasta in salted boiling water until al dente. Drain and set aside.
03 - In a large skillet, melt 3 tbsp butter over medium heat. Add onion, celery, and garlic; sauté 3–4 minutes until softened.
04 - Sprinkle in flour and cook for 1 minute, stirring constantly.
05 - Gradually whisk in chicken broth and milk. Bring to a simmer and cook 3–4 minutes until thickened.
06 - Stir in thyme, parsley, salt, and pepper. Add shredded chicken, peas, carrots, and corn; cook for 2 minutes until heated through.
07 - Add cooked pasta and mix until well combined.
08 - Pour the mixture into the prepared baking dish.
09 - In a small bowl, combine panko, Parmesan, and melted butter. Sprinkle evenly over casserole.
10 - Bake for 20–25 minutes until bubbling and golden on top.
11 - Let cool 5 minutes before serving.