Preheat oven to 350F and lightly grease a 9x13 inch casserole dish.
Cook the egg noodles: boil according to package directions but drain them 2 minutes early so that they don't get too soft. Set aside.
Cook the vegetables: In a large skillet, melt the butter over medium heat. Add the onion, celery and carrots. Cook for 7-10 minutes or until the vegetables are tender.
Season and add flour: stir in the salt, pepper, garlic powder and thyme. Sprinkle the flour over the vegetables and stir well. Cook for 2-3 minutes to remove the raw flour taste.
Add liquids and thicken: slowly whisk in the chicken broth and milk. Bring to a gentle simmer and cook until the mixture thickens and becomes bubbly.
Make it creamy: remove the skillet from the heat. Stir in the sour cream and 1 ½ cups cheddar cheese until smooth and melted.
Combine everything: in your skillet (if large enough) or in a large bowl, combine everything: mix together the cooked noodles, chopped turkey, and the creamy vegetable sauce.
Assemble the casserole: transfer the mixture to the prepared baking dish. Sprinkle the remaining 1 ½ cups of cheddar cheese evenly over the top.
Bake: bake for 30 minutes or until the casserole is hot and the cheese is melted and bubbly.
Serve: let it rest for a few minutes, then garnish with parsley (optional) and serve.