Step 1Mince the mushrooms or process in a food processor until finely and evenly minced.
Step 2Heat a large skillet over high heat until hot. Add the olive oil and butter and melt quickly, then stir in the mushrooms and salt. Cook, stirring frequently, until the liquid released from the mushrooms evaporates. Stir in the shallot and continue to cook just until the mushrooms begin to darken, another minute or so.
Step 3Stir in the wine, careful of the steam as it is very hot and could burn. Stir occasionally until the wine evaporates, 2 to 3 minutes, then continue to cook until the mushrooms are very dry and richly colored, 6 to 8 minutes. Remove from heat and spread the mushrooms out on a rimmed baking sheet to cool completely. This makes a scant cup of duxelles.
Step 1In a large, wide, heavy-bottom saucepan, combine the butter, salt and water and bring to a simmer over high heat. Remove from heat and quickly stir in the flour (stir quickly or the flour lumps will cook). Return the pan to low heat and cook, stirring frequently, for about 3 minutes to cook the flour slightly and rid the mixture of any starchy, floury taste.
Step 2Remove the pan from the heat and stir in the eggs, 1 at a time, until thoroughly combined (stir in the eggs vigorously and quickly as the heat from the pan can scramble them).
Step 3Stir in the duxelles. Remove the mixture from the pan to a medium bowl set over a bowl of ice water (an ice bath).
Step 4When the mixture has chilled and thickened, fold in the sour cream, 1 tablespoon at a time. Continue to gently stir the quenelle base until well chilled.
Step 5Bring a large pot of lightly salted water to a gentle simmer. Gather 2 spoons (standard spoons, not soup spoons) and place 1 in each hand. Dunk the spoons into the simmering water until slightly warmed, then scoop a mounded spoonful of quenelle dough in 1 spoon. Using the other spoon, work the mixture, smoothing and shaping with each spoon, to form a quenelle.
Step 6With the formed quenelle sticking to 1 spoon, lower that spoon gently into the simmering water and shake, gently, until the quenelle releases and drops to the bottom of the pot. Continue until all the dough is used; depending on the size of the spoons, you should have about 20 quenelles.
Step 7The quenelles will float to the surface and puff up as they cook. Continue to simmer gently until the quenelles are puffed and just firm to the touch, about 5 minutes depending on their size (cut one open and test it; it should be cooked through and set).
Step 8Gently remove the cooked quenelles with a slotted spoon and drain on a slanted baking sheet (angle the sheet so any liquid from the quenelles drains away). Continue until all of the quenelles are cooked, and set them aside until cooled. Place the cooled quenelles in a baking dish, cover and refrigerate until well chilled. The quenelles can be made ahead up to this point and refrigerated up to 3 days.
Step 1In a large (12- to 14-inch) saute pan, combine the vegetable broth, wine, shallot and garlic. Bring the mixture to a simmer over high heat and cook until the mixture reduces by half (or the mixture comes no more than one-fourth inch up the side of the pan). Reduce the heat to a very gentle simmer.
Step 2Gently place the mushroom quenelles in the pan, leaving enough space between them to allow them to steam. Cover the pan tightly and steam the quenelles just until warmed through, about 3 to 5 minutes.
Step 3Gently remove the warmed quenelles from the pan with a spatula, fork or slotted spoon and set aside in a warm place.
Step 4Add the blanched vegetables to the pan and cover. Steam the vegetables just until warmed through, about 1 minute. Strain the vegetables with a slotted spoon and mound on each of 6 warmed plates, and garnish each mound with a sprinkling of fleur de sel and cracked pepper. Divide the quenelles between the plates, about 3 per serving (you may not use all of the quenelles).
Step 5There should still be a thin film of liquid on the bottom of the warm pan (a few tablespoons); if it is mostly evaporated, add a little more vegetable broth and wine and swirl around the pan until warm (conversely, if there is too much liquid, drain the liquid until you have about 3 tablespoons in the pan).
Step 6Add the cold butter, a few pieces at a time and swirl the pan until they melt (do not whisk or stir, as the sauce could pick up metal residue from the pan and discolor). When the butter is almost melted, add more pieces, continuing until all of the butter is incorporated. The sauce will thicken as the butter is added. (Adding, or "mounting" the sauce with butter can be very tricky as the pan and broth must be just warm -- neither too hot nor too cold -- otherwise the butter will separate. If the butter does separate, whisk in a little cold cream to bind the sauce together for service.)
Step 7Drizzle a little of the sauce over each plate and serve immediately.