Cook's Note: If turkey fat is not available, substitute with bacon fat or canola oil.
Position a rack in the lower third of the oven and preheat to 250 degrees F. If using convection, preheat to 225 degrees F and set the rack in the middle of the oven.
Spread the Chive Biscuit cubes out on a baking sheet. Toast in the oven until lightly browned, 35 to 45 minutes. Set aside. Increase the oven temperature to 375 degrees F. Take the remaining toasted scraps from the biscuits and place in the food processor. Pulse until crumbs form. Set aside.
In a medium saucepan over medium-low heat, cook the chicken stock and chestnuts until warm. Keep over low heat.
In a deep saute pan or sautoir over medium-high heat, heat the turkey fat, and then add the onions. Cook, stirring occasionally, until the onions are soft and become translucent. At this stage, caramelization of the onions will start. Caramelize the onions lightly, about 10 minutes. Add the parsnips and celery and sweat until tender, 5 to 6 minutes. Remove the vegetables to a large bowl and set aside.
Place the pan back on the heat and add 1 tablespoon butter, and then add the mushrooms. Once the mushrooms have browned and are tender, add the aromatics - parsley, sage and thyme - and cook until the aroma of the herbs is present, about 5 minutes. Transfer the mushroom mixture to the bowl with the cooked vegetables. Season with salt and pepper.
Add the toasted biscuit cubes, stock and chestnuts. Add the eggs and stir to combine.
Butter a large casserole dish with the remaining 1 tablespoon butter and transfer the dressing to the dish. Spread the reserved biscuit crumbs over the top of the dressing. Cover with aluminum foil (or the supplied lid). Bake for 30 minutes, then remove the foil and bake until the dressing is lightly browned, 15 to 20 more minutes.
Preheat the oven to 350 degrees F. Line 2 baking sheets with parchment paper.
In a stand mixer, add the flour, baking powder, sugar, salt and baking soda to the bowl. At a slow speed, cut the fat into the flour until coarsely mixed. Add the chives, and then slowly add the buttermilk. Continue mixing until just combined.
Dust the work surface with flour and turn the dough out onto the floured board. Roll the dough out until it is 1-inch thick. Using a 2 1/2-inch biscuit cutter, cut out 12 rounds. Place onto the prepared baking sheets, brush with buttermilk and sprinkle with celery salt. Break up the scraps of the leftover dough and place on second baking sheet. Bake until golden brown, 25 to 30 minutes. Yield: 12 biscuits with scraps, or 15 with re-rolling