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1
Preheat oven to 450F.
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2
Pierce potatoes with fork.
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3
Bake until tender, about 1 hour.
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4
Cool slightly; cut potatoes in half and scoop pulp into large bowl.
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5
Mash potatoes well.
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6
Mix in egg yolk, salt, pepper and nutmeg.
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7
Mix in enough flour to form firm, slightly elastic dough.
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8
Turn out onto lightly floured surface.
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9
Divide dough into 4 equal portions.
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10
Gently roll 1 portion between hands and work surface to form 1/2-inch-thick rope about 18 inches long.
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11
Cut crosswise into 1/2-inch pieces.
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12
Roll each piece over tines of dinner fork to make grooves in gnocchi.
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13
Arrange gnocchi in single layer on floured baking sheet.
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14
Repeat with remaining 3 portions of dough.
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15
Working in small batches, cook gnocchi in large pot of boiling salted water until gnocchi rise to top and are tender, about 5 minutes.
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16
Using slotted spoon, transfer gnocchi to large baking pan.
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17
(Gnocchi can be prepared ahead.
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18
Let stand 1 hour at room temperature, or cover and refrigerate up to 2 days.)
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19
Cook butter and olive oil in heavy large skillet over medium heat until butter begins to brown, about 2 minutes.
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20
Add mushrooms and shallots and saute until golden brown, about 10 minutes.
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21
Add stock and sage; simmer until liquid is slightly reduced, about 8 minutes.
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22
Season to taste with salt and pepper.
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23
Add gnocchi to sauce in skillet and cook until heated through, about 1 minute.
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24
Add arugula and stir until wilted.
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25
Divide equally among bowls.
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26
Drizzle with truffle oil, if desired.
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27
Sprinkle with cheese.
