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 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 eliminate raw flour taste.
05 - Gradually whisk in chicken broth and whole milk, ensuring no lumps form. 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 dried thyme, parsley, black pepper, and salt. Add shredded chicken, frozen peas and carrots, and frozen corn. Cook for 2 minutes until everything is heated through.
07 - Add cooked pasta to the skillet and toss until evenly coated with the creamy chicken mixture.
08 - Pour the pasta mixture into the prepared baking dish, spreading it into an even layer.
09 - In a small bowl, combine panko breadcrumbs, grated Parmesan cheese, and melted butter. Mix until evenly combined.
10 - Sprinkle the breadcrumb mixture evenly over the pasta. 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 rest for 5 minutes before serving to allow the sauce to set slightly.