Make the turkey stock. Put carcass in a large pot, add onion, carrot, celery, peppercorns, parsley and a generous pinch of salt. Cover with at least 8 cups of water, bring to a boil, then turn down, cover partially and simmer for 1-½ to 2 hours, skimming any impurities that float to the top. Strain stock in a tea towel-lined strainer and set aside. Pull any meat off all those bones, especially the neck, and add to your shredded leftovers. If needed, when the stock cools a bit, skim off any fat (which you can use to sauté the vegetables if you want, instead of butter).
In a large stock pot or soup pot, melt 2 tablespoons unsalted butter (or turkey fat) and add the onion, carrot and celery. Add a sprinkle of salt and cook until the onions are softened, about 10 minutes, stirring frequently. Add the garlic and cook until fragrant, about a minute, be careful not to burn.
Add the chopped parsley, celery leaves, sage, thyme (or your seasonings of choice), and salt and pepper. Stir and cook for a minute or two. Add 8 cups, more or less, of the turkey stock and bring to a simmer. With lid partially on, simmer until carrots and celery are tender, stirring occasionally, 15 – 20 minutes.
Add the frozen egg noodles and simmer another 20 minutes. Stir in the shredded turkey. Heat through and season with salt and pepper to taste.
Notes
Leftovers of this soup taste great! Just keep in mind the noodles will continue to soak up the broth so you may need to add more when you reheat.