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1
Preheat the oven to 425.
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2
Heat the goose fat in a large, nonstick skillet over moderate heat until hot but not smoking.
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3
Add the mushrooms, season lightly with sea salt, and saute just until the mushrooms begin to give up their juices, about 5 minutes.
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4
Reduce the heat, add the thyme, garlic, half the parsley and half the walnuts.
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5
Cook, stirring regularly, for about 1 minute more.
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6
Season generously with sea salt and pepper.
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7
Transfer the mixture to a 9-inch tarte Tatin pan or oven proof, nonstick skillet.
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8
Place the Tatin pan or skillet on a baking sheet.
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9
Remove the pastry from the refrigerator and place it on top of the mushroom mixture, gently pushing the edges of the pastry down around the edge of the pan.
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10
Bake the tart on the baking sheet in the center of the oven until the pastry is golden, 20 to 25 minutes.
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11
Meanwhile, in a small jar, season the lemon juice with sea salt.
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12
Cover and shake to dissolve the salt.
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13
Add the walnut oil, cover, and shake again.
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14
Taste for seasoning.
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15
Remove the baking sheet from the oven.
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16
Immediately invert a serving platter with a lip over the skillet.
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17
Quickly but carefully invert the tart on top of the serving platter so the mushrooms are on top and the pastry is on the bottom.
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18
Should any mushrooms stick to the bottom of the pan, remove them and place them back into the tart.
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19
Sprinkle the mushrooms with the remaining tablespoon of parsley and the remaining walnuts.
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20
Drizzle with 2 teaspoons of the walnut oil vinaigrette (reserve the remaining vinaigrette for another use); season with fleur de sel.
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21
Serve warm, cut into wedges.
