Ingredients

  • 3 cups all-purpose flour plus additional for kneading
  • 1 cup water
  • 1 large egg
  • 2 teaspoons vegetable oil
  • 1 teaspoon salt
  • 1 1/2 pound russet (baking) potatoes
  • 6 ounces coarsely grated extra-sharp white Cheddar (2 1/4 cups)
  • 1/4 teaspoon salt
  • 1/4 teaspoon black pepper
  • 1/8 teaspoon ground nutmeg
  • 1 medium onion, halved lengthwise and thinly sliced crosswise
  • 1 stick (1/2 cup) unsalted butter
  • Special equipment: a 2 1/2-inch round cookie cutter
  • Accompaniment: sour cream

Method

  • Put flour in a large shallow bowl and make a well in center.
  • Add water, egg, oil, and salt to well and carefully beat together with a fork without incorporating flour.
  • Continue stirring with a wooden spoon, gradually incorporating flour, until a soft dough forms.
  • Transfer dough to a lightly floured surface and knead, dusting with flour as needed to keep dough from sticking, until smooth and elastic, about 8 minutes (dough will be very soft).
  • Invert a bowl over dough and let stand at room temperature 1 hour.
  • Peel potatoes and cut into 1-inch pieces.
  • Cook potatoes in a large saucepan of boiling salted water until tender, about 8 minutes.
  • Drain potatoes, then transfer to a bowl along with cheese, salt, pepper, and nutmeg and mash with a potato masher or a handheld electric mixer at low speed until smooth.
  • When mashed potatoes are cool enough to handle, spoon out a rounded teaspoon and lightly roll into a ball between palms of your hands.
  • Transfer ball to a plate and keep covered with plastic wrap while making 47 more balls in same manner (there will be a little filling left over).
  • Cook onion in butter in a 4- to 5-quart heavy saucepan over moderately low heat, stirring occasionally (stir more frequently toward end of cooking), until golden brown, about 30 minutes.
  • Remove from heat and season with salt and pepper.
  • Halve dough and roll out 1 half (keep remaining half under inverted bowl) on lightly floured surface (do not overflour surface or dough will slide instead of stretching) with a lightly floured rolling pin into a 15-inch round (1/8 inch thick), then cut out 24 rounds with lightly floured cutter.
  • Holding 1 round in palm of your hand, put 1 potato ball in center of round and close your hand to fold round in half, enclosing filling.
  • Pinch edges together to seal completely.
  • (If edges don't adhere, brush them lightly with water, then seal; do not leave any gaps or pierogi may open during cooking.)
  • Transfer pierogi to a lightly floured kitchen towel (not terry cloth) and cover with another towel.
  • Form more pierogies in same manner.
  • Bring a 6- to 8-quart pot of salted water to a boil.
  • Add half of pierogies, stirring once or twice to keep them from sticking together, and cook 5 minutes from time pierogies float to surface.
  • Transfer as cooked with a slotted spoon to onion topping and toss gently to coat.
  • Cook remaining pierogies in same manner, transferring to onions.
  • Reheat pierogies in onion topping over low heat, gently tossing to coat.