Preheat the oven to 400°F and spray some oil into a 9×13 inch baking dish.
Melt butter in a large pan, cook onions, celery, carrots, and apples, and season with salt. Stir occasionally until the vegetables start to brown, which is about 10-12 minutes. Let it cool.
Mix together egg, chicken, milk, mustard, bread crumbs, poultry seasoning, pepper, and salt in a large bowl. Add the carrot mixture and stir it gently.
Transfer to the baking dish and shape into a loaf. Bake for 30 minutes, then reduce the temperature to 375°F and bake for 25 minutes more or until the centre is cooked.
Serve it with mashed potatoes and asparagus. You can choose different side dishes like salad or rice. Enjoy!