01 - Preheat oven to 375°F. Grease a 9x13-inch baking dish with butter or cooking spray.
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 - 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 and form a paste.
05 - Slowly whisk in chicken broth, followed by whole milk. Bring mixture to a gentle simmer, whisking frequently. Cook for 3–4 minutes until sauce thickens enough to coat the back of a spoon.
06 - Add dried thyme, parsley, black pepper, and salt. Stir well to incorporate herbs evenly throughout the sauce.
07 - Stir in shredded chicken, frozen peas and carrots, and frozen corn. Simmer for 2 minutes until everything is heated through.
08 - Add cooked pasta to the skillet. Gently fold until pasta is evenly coated with sauce and ingredients are well distributed.
09 - Pour the entire mixture into the prepared 9x13-inch baking dish. Spread evenly with a spatula.
10 - In a small bowl, combine panko breadcrumbs, grated Parmesan cheese, and melted butter. Mix until breadcrumbs are evenly coated.
11 - Sprinkle breadcrumb mixture evenly over the casserole. Bake for 20–25 minutes until sauce is bubbling around edges and topping is golden brown.
12 - Remove from oven and let rest for 5 minutes. This allows the sauce to set slightly for easier serving.