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 1-1/2 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.