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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Coat a 9-by-13-inch baking dish with butter; set aside.
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3
Spread bread cubes in a single layer on a baking sheet and toast until dry and lightly browned, about 15 minutes.
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4
Remove from the oven, let cool slightly, and transfer to a large bowl; set aside.
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5
Meanwhile, heat 1 tablespoon of the oil in a large frying pan over medium-high heat until shimmering.
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6
Add mushrooms and cook, stirring rarely, until browned, about 6 minutes.
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7
Remove from heat and transfer to the bowl with the bread cubes; set aside.
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8
Using a vegetable peeler, peel celery root and cut into 1/2-inch cubes.
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9
(Peel and cut the celery root at the last minute, because it will oxidize when exposed to air.)
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10
Return the frying pan to medium heat and add butter and remaining 2 tablespoons oil.
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11
When butter has melted, add celery root, onion, sage, and thyme and season generously with salt and pepper.
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12
Cook, stirring occasionally, until onion just begins to soften, about 6 to 8 minutes.
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13
Transfer vegetables to the bowl of bread cubes and mushrooms.
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14
Evenly drizzle wine around the bowl, add egg, and stir to evenly coat bread cubes and vegetables.
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15
Drizzle half of the mushroom broth over the mixture and stir to evenly combine.
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16
Drizzle in the remaining broth and stir until all bread cubes are evenly moistened.
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17
Season with additional salt and pepper as needed.
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18
Transfer stuffing to the prepared baking dish, slightly compacting it.
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19
Cover with foil and bake for 30 minutes.
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20
Remove foil and continue to bake until bread cubes on top are crispy and browned, about 30 minutes more.
