Cut the loaf of bread into ½-inch cubes. Spread them out in one layer on a baking sheet and toast at 350ºF for 5 to 15 minutes, until lightly colored. Set aside.
If using whole chestnuts, increase the oven temperature to 400°F. Using a sharp paring knife, place the flat end of the chestnut on a cutting board and carefully cut an “X” on the top of each one, just cutting through the shell. Place on a baking sheet and roast at 400ºF for 25 minutes until the skin curls up away from the nut. (Alternatively, you can air-fry the chestnuts at 370°F for 12 to 15 minutes.) When the shells of the chestnuts are darker in color and the cut skin has peeled away from the center, remove them from the oven. The shell should peel away easily at this point. If it doesn’t, continue to roast for a few more minutes. Allow the chestnuts to cool so that you can handle them and then peel away the shells and chop the nuts into small pieces.
If you are using cooked and peeled chestnuts, chop into small pieces and set aside.
Pre-heat your slow cooker using the SAUTE setting or pre-heat a large skillet on the stovetop. Brown the sausage until it is no longer pink. Remove the sausage from the cooker with a slotted spoon and set it aside. Melt the butter in the cooker along with any leftover fat from the sausage. Add the onions and celery and sauté for a few minutes, until they start to soften. Add the mushrooms and sauté for another 5 minutes, until the vegetables have cooked through.
Add the cooked sausage, chopped chestnuts and parsley to the cooker and toss everything to evenly combine the ingredients. Add the stuffing mix and combine well. Stir the toasted bread cubes into the cooker batches, combining them with the other ingredients carefully.
Mix the eggs and the Parmesan cheese together in a small bowl and stir into the stuffing mixture. Gradually add 2 cups of the chicken stock, stirring to evenly combine. Add more of the remaining stock as needed – the stuffing should look and feel moist. Season with salt and freshly ground black pepper.
*If you are making this stuffing ahead or time, hold this mixture in the refrigerator until you are ready to cook. Remove the stuffing mixture from the refrigerator 30 minutes before you want to start slow cooker to bring the ingredients to room temperature. In the meantime, pre-heat the slow cooker on HIGH for 30 minutes. Pour another ½ cup of chicken stock evenly over top the stuffing in the cooker and proceed with the recipe.
Slow-cook on HIGH for 3 to 4 hours. Check the stuffing after 2½ hours and if it starts to dry out, add a little more chicken stock. Keep on KEEP WARM until you are ready to serve.