1 (14- to 16-ounce) brioche loaf, torn into 1-inch pieces
8 tablespoons unsalted butter (1 stick), plus more for the pan
1 large onion, diced
2 large celery ribs, thinly sliced
1/2 cup diced fennel (about 1/2 small fennel bulb)
1 1/2 teaspoons kosher salt, plus more as needed
1 tablespoon fresh thyme, chopped
1 tablespoon fresh sage, chopped
5 ounces roasted, peeled chestnuts, roughly chopped
1/2 teaspoon ground black pepper
2 1/2 cups turkey or chicken stock
2 large eggs
1/4 cup chopped parsley
Arrange brioche pieces in a single layer on a rimmed baking sheet. Let them dry out overnight, or place them in a 200-degree oven for an hour or two. (They’ll be ready when they feel stale to the touch but haven’t taken on any color.)
Heat oven to 375 degrees, and butter a shallow, 2-quart casserole or gratin dish.
On a pot on the stove or in the microwave, melt 4 tablespoons butter.
Put bread in the prepared baking dish and toss with melted butter. Bake until golden and toasted, 8 to 10 minutes.
Let cool, then transfer toast to a large bowl. (Don’t wash the baking dish; you’ll use it again for the stuffing.)
In a 12-inch skillet, melt remaining 4 tablespoons butter over medium heat. Stir in onion, celery, fennel and 1/2 teaspoon salt; cook until soft and just starting to brown, about 12 minutes.
Stir in thyme and sage, and cook for 1 minute.
Remove from heat and transfer to bowl with brioche. Gently fold in chestnuts and pepper and let cool.
In a medium bowl, whisk together 1 1/2 cups stock, eggs, parsley and remaining 1 teaspoon salt.
Fold gently into bread mixture, then scrape it all back into prepared baking dish.
Drizzle on remaining 1 cup stock until the mixture is moist but not squishy; you may not need all the stock.
Cover dish with foil and bake until lightly springy, about 25 minutes.
Remove foil and continue to bake until golden brown, another 20 to 30 minutes.