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1
In a large bowl, stir together the flours and 1 teaspoon salt.
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2
Beat in the eggs and water with the handle of a wooden spoon, then continue to knead the dough with your hands until a smooth and elastic dough has formed, about 10 minutes.
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3
Wrap the dough in plastic wrap and let stand at room temperature for 30 minutes.
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4
In a medium heavy pot, cover the potatoes with salted water and bring to a boil.
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5
Boil the potatoes until very tender, about 20 minutes.
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6
Strain and reserve the potatoes.
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7
While the potatoes cook, in a large heavy skillet, heat the oil over medium high heat until hot.
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8
Stir in the onion and 1/2 teaspoon each salt and pepper, then cook, stirring occasionally, until golden, about 6 minutes.
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9
Add the potatoes to the skillet with the onion and mash together until smooth.
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10
Season the filling with salt to taste.
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11
Let cool to warm.
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12
Halve the dough, keeping one half wrapped, then roll out the other half of the dough on a floured work surface to a 14-inch round.
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13
Using a 2-to 3-inch round cutter, cut out rounds from the dough.
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14
Place a scant tablespoon of the filling on one half of each round of dough, then moisten the edges of the rounds with water and fold into half moons.
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15
Repeat with the remaining dough and the remaining filling.
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16
In a large pot of boiling salted water, cook the pierogi in batches until tender, about 12 minutes per batch.
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17
Transfer the pierogi with a slotted spoon to an oiled baking sheet.
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18
In a large heavy skillet, heat 2 tablespoons of the butter over medium heat until hot, then brown the pierogi in batches, adding additional butter as needed.
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19
Serve the pierogi with sour cream and chives.
