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 a boil. Add pasta and cook 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 create a smooth paste.
05 - Gradually whisk in chicken broth, followed by whole milk. Bring mixture to a simmer, continuing to whisk. Cook for 3–4 minutes until sauce thickens enough to coat the back of a spoon.
06 - Stir in dried thyme, parsley, salt, and black pepper. Add shredded chicken, frozen peas and carrots, and corn. Cook for 2 minutes until everything is heated through.
07 - Add cooked pasta to the skillet. Fold gently until pasta is evenly coated with sauce and ingredients are well distributed throughout.
08 - Pour the pasta 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 with a fork until butter is evenly distributed and mixture forms coarse crumbs.
10 - Sprinkle the crumble topping 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 casserole rest for 5 minutes before serving. This allows the sauce to set slightly for easier scooping.