Steep the dried porcini in the warm water for about 20 minutes, agitating them a bit to dislodge any sand, letting it sink to the bottom. Lift out the reconstituted mushrooms and pour the soaking liquid through a sieve lined with a coffee filter into a container to remove any sand. Finely chop the mushrooms and set them and the liquid aside.
In a large saucepan, sauté the shallots for a few minutes in 1½ tablespoons butter with a pinch of salt. Set about 3½ ounces (100 g) of the prettiest fresh mushrooms aside for garnish and add the remaining fresh mushrooms to the saucepan and sauté them for 5 minutes. Add the chopped porcini and sauté for 1 minute. Add the sherry, if using. Add the reserved soaking water and the stock. Bring everything to a boil and let simmer for about 20 minutes.
Puree the soup with a hand blender, or in batches in a regular blender, until smooth. Add the remaining
Just before serving, briefly sauté the reserved fresh mushrooms with the hazelnuts in some butter, and season with salt and pepper.
Reheat the soup until just under a boil—no hotter!—stir thoroughly, and divide among four bowls. Garnish the soup with a lick of crème fraîche, the sautéed mushrooms and nuts, and a drop of hazelnut oil.
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