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 until al dente according to package directions. Drain well and set aside.
03 - Melt 3 tablespoons butter in a large skillet over medium heat. Add onion, celery, and garlic. Sauté for 3-4 minutes until vegetables are softened and fragrant.
04 - Sprinkle flour evenly over the vegetables. Stir constantly for 1 minute to cook out the raw flour taste and create a roux base.
05 - Gradually whisk in chicken broth and milk, smoothing out any 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 thyme, parsley, salt, and black pepper. 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 fold gently until evenly coated with sauce and distributed throughout.
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 crumbs are evenly coated.
10 - Sprinkle the breadcrumb mixture evenly over the pasta, covering the surface completely.
11 - Bake for 20-25 minutes until the sauce is bubbling around the edges and the topping is golden brown and crispy.
12 - Remove from oven and let cool for 5 minutes before serving to allow the casserole to set for easier scooping.