- Pre-heat your oven to 250 degrees.
- Spread the bread out onto a large rimmed baking sheet and bake it in your pre-heated oven for 60 minutes. The bread should feel dry and crisp. Remove the baking sheet from your oven and set it aside to cool. Once cooled, transfer the bread to a very large bowl and set aside. The bread can be baked 1-2 days in advance and stored in an airtight container at room temperature.
- Increase your oven temperature to 350 degrees. Lightly coat an 8 X 8-inch baking dish with non-stick cooking spray and set aside.
- In a large skillet over medium heat, melt the butter with the olive oil. Add the celery, fennel and onion and cook, stirring occasionally, for 15 minutes. If the vegetables start to brown, turn down the heat.
- Stir in the salt and black pepper and cook for 2 minutes more. Then remove the skillet from the heat and stir in the chestnuts, apricots, all the fresh herbs and the mashed potatoes. Set aside.
- In a medium bowl, whisk together 1 1/2 cups of the vegetable broth with the egg. Slowly pour this mixture over the bread in the bowl. Toss with a pair of tongs to make sure most of the bread is moistened. Then scrape in the cooked vegetables/herbs, folding gently until all the ingredients are well combined.
- Transfer the stuffing to your prepared baking dish and pat it down lightly, just so it fits evenly in the dish. Pour the remaining 1/4 cup of vegetable broth over top. Then cover the baking dish tightly with aluminum foil and transfer it to your pre-heated oven. Bake for 40 minutes. Carefully remove the aluminum foil and increase your oven temperature to 425 degrees. Bake uncovered for an additional 20 minutes. The top should be golden brown and crisp.
- Remove the baking dish from your oven and set it aside to let the stuffing rest for 10 minutes before serving.
- Leftovers can be stored in an airtight container in your refrigerator for 3-4 days