01 - Preheat oven to 375°F. Grease a 9x13-inch casserole dish with butter or cooking spray.
02 - Bring a large pot of salted water to boil. Add egg noodles and cook according to package directions until al dente. Drain thoroughly and set aside.
03 - Melt 2 tablespoons butter in a large skillet over medium heat. Add chopped onion and minced garlic, sauté for 3-4 minutes until softened and fragrant.
04 - Stir peas and carrots into the skillet with the onion mixture. Cook for 3 minutes, stirring occasionally until heated through.
05 - In a large mixing bowl, combine cooked noodles, chicken, sautéed vegetables, cream of chicken soup, sour cream, milk, cheddar cheese, dried thyme, salt, and black pepper. Stir until evenly combined and no dry pockets remain.
06 - Pour the casserole mixture into the prepared dish and spread evenly with a spatula.
07 - In a separate bowl, combine panko breadcrumbs, melted butter, Parmesan cheese, and fresh parsley if using. Mix with a fork until breadcrumbs are evenly coated.
08 - Sprinkle the crumble topping evenly over the casserole mixture, creating an even layer.
09 - Bake for 30-35 minutes until the topping is golden brown and the casserole is bubbling around the edges.
10 - Remove from oven and allow to rest for 5 minutes to set before serving.