Tangy Apple Chicken Loaf

Preheat oven to 400 degrees F (200 degrees C).
Melt the butter in a skillet over medium heat, and cook the celery and onion with 1/2 teaspoon of salt, stirring occasionally, until the onions are soft and browned, about 12 minutes. Stir in the carrot, and cook for 5 more minutes; stir in the apple, and remove the mixture from the heat.
In a mixing bowl, lightly mix together the ground chicken, egg, 1/3 cup of applesauce, milk, bread crumbs, poultry seasoning, sage, 1/2 teaspoon of salt, and black pepper. Mix in the cooked vegetable-apple mixture. Form the mixture into a loaf, and place into the center of a 9x12-inch baking dish.
In a bowl, mix together 1/3 cup of applesauce, the Dijon mustard, and honey into a smooth mixture, and spread the glaze over the top and sides of the meat loaf.
Bake in the preheated oven for 30 minutes, then reduce the temperature to 375 degrees F (190 degrees C) until the internal temperature of the loaf is 180 degrees F (80 degrees C), about 30 more minutes. Allow the loaf to rest for 5 minutes before serving.