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1
Preheat the oven to 375 degrees.
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2
In a small pot, bring 3 cups water to a boil.
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3
Remove from the heat, add the porcini and let sit for 30 minutes.
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4
Strain through a fine-mesh sieve set over a bowl and set aside.
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5
Rinse and chop the mushrooms.
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6
Fill a large bowl with cold water and add all but 2 tablespoons of the lemon juice.
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7
Peel, quarter and cut the celery root into 1/4-inch-thick slices and add to the water.
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8
Place a large, ovenproof pot over medium-high heat.
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9
Heat 1/4 cup of the oil, then add the leek, onion, garlic, thyme sprigs, bay leaf and a pinch each of salt and pepper.
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10
Saute until the onions are translucent.
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11
Drain the celery root and add it to the onion mixture with the reserved mushroom broth and milk.
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12
Bring to a boil, cover tightly and transfer to the oven.
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13
Braise until the celery root is tender, about 40 minutes.
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14
Remove the bay leaf and thyme sprigs.
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15
Using a blender or immersion blender, carefully blend the mixture to a smooth puree.
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16
(For a finer texture, strain through a fine-mesh sieve.)
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17
Place the pot over low heat and simmer until it reaches the consistency of porridge.
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18
Add the remaining 2 tablespoons lemon juice and season to taste with salt and pepper.
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19
In a large skillet over medium-high heat, heat the remaining 1/4 cup oil.
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20
Add the fresh mushrooms and saute until the water releases and evaporates.
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21
Add the rehydrated porcini and chopped thyme.
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22
Season with salt and pepper to taste.
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23
To serve, place a few spoonfuls of the puree on each plate and drizzle with truffle oil, if using.
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24
Top with mushrooms and Parmesan.
