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1
In a small heatproof bowl, cover the dried porcini with the boiling water and let stand until the mushrooms are softened, about 25 minutes.
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2
Drain, rinse off any grit and finely chop the porcini mushrooms; discard the soaking liquid.
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3
In a medium skillet, heat 1 tablespoon of the butter.
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4
Add the chopped shallots and cook over moderate heat, stirring occasionally, until softened, about 4 minutes.
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5
Add the chopped porcini, season lightly with salt and black pepper and cook until fragrant, about 3 minutes.
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6
Let cool, then stir in the Gruyere cheese.
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7
Meanwhile, in a large bowl, toss the potatoes and onion with 1 tablespoon of kosher salt and let stand for 15 minutes.
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8
Working with a handful at a time, squeeze as much of the liquid as possible from the potato-and-onion mixture into a large bowl.
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9
Transfer the squeezed potatoes and onion to a work surface.
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10
Let the liquid in the bowl stand for 5 minutes to allow the potato starch to settle on the bottom.
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11
Pour off the liquid, leaving only the starch in the bowl.
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12
Return the potatoes and onion to the bowl and toss with the starch.
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13
Season generously with black pepper.
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14
Preheat the oven to 375.
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15
In a 12-inch nonstick ovenproof skillet, melt the remaining 2 tablespoons of butter in 2 tablespoons of the olive oil.
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16
Remove from the heat.
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17
Add half of the potato-onion mixture, pressing it into an even layer with a spatula.
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18
Spoon the porcini-Gruyere mixture over the potatoes, leaving a 1-inch border all around the outside.
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19
Top with the remaining potato-onion mixture and press to seal the potato cake.
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20
Cook the potato cake over moderately high heat, shaking the pan occasionally, until the bottom is golden brown, about 7 minutes.
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21
Carefully slide the potato cake onto a large, flat plate.
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22
Invert the skillet over the cake and, using oven mitts or towels, carefully invert the cake back into the skillet.
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23
Drizzle 2 tablespoons of olive oil around the edges and cook the potato cake for 5 minutes.
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24
Transfer the skillet to the oven and bake the potato cake for 30 minutes, until it is cooked through and golden on the bottom.
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25
Slide the potato cake onto a plate.
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26
In a large bowl, whisk the remaining 1/4 cup of olive oil with the lemon juice and walnut oil.
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27
Season with salt and black pepper.
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28
Add the baby greens and tomatoes and toss again.
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29
Cut the potato cake into wedges and sprinkle them with the truffle salt, if using.
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30
Serve with the salad.
