PreparationMake dough: 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. Make filling while dough stands: 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). Make onion topping: 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. Form and cook pierogies: 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. Cooks' notes: