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1
Make the dumpling dough: Lightly beat the yogurt, egg, and salt together with an electric mixer.
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2
Slowly add the flour and continue beating until smooth.
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3
Knead the dough on a floured surface until smooth.
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4
Add water, a teaspoon at a time, only if needed to smooth out the dough; it should feel like pizza dough.
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5
Form the dough into a ball, wrap it in a kitchen towel, and set aside in a cool place for 2 hours.
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6
Meanwhile, make the filling: Peel and quarter the potato.
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7
Cover with cold water, bring to a boil, lower the heat, and simmer until it offers no resistance when pricked with a fork, 15 to 20 minutes.
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8
Drain and set aside.
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9
Melt 2 tablespoons of the butter in a skillet over very low heat.
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10
Whip the potato with the butter until it is fluffy and creamy.
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11
Set aside.
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12
Melt the remaining 2 tablespoons butter in the skillet over medium heat.
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13
Add the onion and saute until translucent, 5 minutes.
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14
Add the mushrooms, salt, and pepper and saute until the mushrooms are tender and any liquid exuded by the vegetables evaporates, about 5 minutes.
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15
Remove the mixture from the skillet and set aside to cool slightly.
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16
Mix the vegetables into the mashed potatoes.
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17
Bring a large pot of salted water to a boil.
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18
Meanwhile, roll out the dough 1/8 inch thick.
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19
Cut into 3-inch rounds with a cookie cutter or the rim of a glass.
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20
Put about 1 heaping teaspoon of the mashed potato filling on one side of the circle.
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21
Fold the other side over the filling so the pierogi looks like a half moon.
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22
Press the edges together with your thumb and index finger, sealing the pierogi.
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23
Lower the heat so the boiling water simmers gently.
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24
One at a time, add the pierogis to the water in batches of 4 to Simmer until cooked through, 5 to 7 minutes.
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25
Remove with a slotted spoon and drain on paper towels.
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26
Keep warm while cooking the remaining pierogis.
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27
Serve with fried onions or sour cream.
