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1
Make filling: place dried mushrooms in a small bowl and add 1/4 cup boiling water.
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2
Let soften, about 10 minutes.
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3
Strain, reserving liquid and mushrooms separately.
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4
In a large skillet, heat oil over medium heat.
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5
Add onion and cook, stirring often, until golden but not brown, about 5 minutes.
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6
Add button mushrooms and cook, stirring, until mushrooms have released their liquid, about 10 minutes.
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7
Sprinkle with salt and pepper and drain liquor.
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8
In a food processor, combine both kinds of mushrooms.
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9
Spoon in porcini liquid, leaving behind any silt in bottom of bowl.
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10
Pulse together until finely ground but not pasty: about 3 or 4 pulses.
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11
Add salt and pepper to taste and set aside.
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12
Make dough: In a small bowl, combine egg yolk, oil and 1 1/2 cups lukewarm water and whisk 1 minute.
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13
Place flour in a large bowl and make a well in center.
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14
Add a third of the egg mixture and lightly mix in with fingers or a fork.
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15
Repeat 2 more times.
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16
Using hands, fold dough together until soft: if crumbly, gently work in more water; if sticky, add flour.
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17
Transfer to a lightly floured board and knead 3 minutes.
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18
Form into a ball, transfer to a bowl and refrigerate 45 minutes.
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19
Lightly flour a work surface and a pan or board for the finished dumplings.
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20
Divide dough into 3 sections.
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21
Using a well-floured rolling pin, roll each section out until very thin and in a rough rectangle.
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22
Use tip of a sharp knife to cut dough into 1 1/2-inch squares.
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23
Drain any excess liquid from filling.
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24
Place 1/2 teaspoon filling in center of each square.
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25
Fold squares in half to form triangles, sealing filling inside.
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26
(If dough is becoming dry, lightly moisten fingertips to seal.)
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27
Pinch the 2 opposing corners together to seal tightly.
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28
Place on floured surface: do not stack.
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29
Repeat with remaining dough and filling.
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30
In a large pot of salted boiling water, cook dumplings until they float, 2 to 4 minutes.
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31
Drain and serve in hot borscht, or just with sour cream.
