Place torn bread on two large sheet pans and bake in the preheated oven to dry. Let dry for approximately 1 hour, switching racks halfway through. Once cooled on the counter, the bread cubes should feel very dry and brittle.
10 cups bread (about 1 loaf)
Over medium heat, melt butter in a large skillet. Add chopped onion and celery. Sauté over medium heat until onions are translucent and slightly tender, 6-8 minutes. Once cooked, remove from the heat and allow to cool slightly.
1 ½ cups chopped onion, 8 tablespoons (1 stick) unsalted butter, 1 ½ cups chopped celery
Place cooled bread in a large bowl and add minced herbs, cooled onion mixture, chestnuts, salt, and black pepper. Stir to combine and add the egg to the stock. Pour half of the stock over the bread. No liquid should be sitting at the bottom of the bowl. Let the bread soak up the liquid and add the remainder.
2 tablespoons minced fresh sage (or 1 tablespoon dry), 1 tablespoon minced fresh thyme (or 1 ½ teaspoons dry), ½ tablespoon minced fresh rosemary (or ¾ teaspoon dry), 1 teaspoon kosher salt, 1 teaspoon freshly ground black pepper, 1 large egg, 1 ½ cups roasted chestnuts, 1 quart turkey/chicken/vegetable stock
Pour into a 6-quart slow cooker and set to low. The stuffing will take 5-6 hours and become slightly puffed and set around the edges.