Make stock first by placing the onion, carrot, and celery into a large pot with the oil. Cover and cook over medium heat until the vegetables are soft and translucent, about 5-10 minutes. Add the turkey bones and cover with water until all the bones are submerged. Bring to a boil, then reduce heat to a simmer, cover, and cook for 4-8 hours on the stovetop.

Strain the stock through a fine mesh strainer and into a new container, discard the solids. Return this stock to the stove and reduce to 4 cups. At this point, you can chill the stock by placing the container into a bath of ice to rapidly chill it and refrigerate until needed. Otherwise, continue by preparing fresh vegetables.

Heat the butter in a large pan over medium-high heat. When hot, add the onion, carrot, and celery. Cook until the vegetables are soft and translucent, about 5-10 minutes. Add the garlic and cook until fragrant, about 1 minute more. Add the bay, sage, thyme, and celery salt. Cook, stirring until fragrant, about 3 minutes. Add in all of the reserved turkey meat and cook until hot, seasoning to taste with salt and pepper.

Finally, when the turkey is hot, add in the 4 cups of stock (reheat it first if you made it a day or more before). Season to taste again with salt and pepper, stir in parsley, and serve.