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1
Set a large pot over medium heat and add the butter. When the butter begins to sizzle and foam, add the carrots, onion, mushrooms, and celery root. Season with a pinch of salt and freshly ground black pepper and turn the heat up to high. Stirring regularly, cook the vegetables until they soften and begin to slightly caramelize around the edges (about 10 minutes). If you are concerned that the butter is beginning to burn, turn the heat down to medium.
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2
When the vegetables look soft and slightly caramelized, add the vermouth. Stir to deglaze the bottom of the pot. Cook until the vermouth has reduced by about half.
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3
Add the chicken stock and season with another pinch salt and freshly ground black pepper. Taste the broth and adjust the seasoning as necessary.
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4
Remove the pot from the heat. Add the shredded chicken along with the diced foie gras (if using). Stir gently. Next, shave the black truffle as thinly as possible and add the shaved truffle to the soup.
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5
Preheat the oven to 425u00b0 F. Fill four ramekins or ovenproof bowls with the soup. Lightly dust a work surface with flour. Take a sheet of puff pastry and roll it out with a rolling pin until it spreads to just less than twice its original size. Use flour as necessary to make sure the puff pastry doesn't stick to the surface you're rolling on. Cut out circles of puff pastry that will fit over the top of your ramekins or ovenproof bowls. If you need to roll out the second sheet of puff pastry to cut enough circles of dough for your bowls/ramekins, please do so.
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6
In a small bowl, mix the egg yolk with a tablespoon of water. Drape the circles of puff pastry over the top of each ramekin/bowl and press the edges of the pastry down over the rim. Using a pastry brush, lightly brush the tops of the puff pastry with the egg wash.
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7
Cook the ramekins/bowls of soup for 15 to 20 minutes, until the puff pastry is fully cooked and looks golden brown. Check the puff pastry after 10 minutes. If the crust looks like it's getting too dark, lower the hear to 375u00b0 F. When the puff pastry looks crispy and golden, remove the soup from the oven and serve immediately.
