After work, I used to hang out at the Eastern European coffee shop Veselka, which is known as much as an after-hours chef hangout as it is for its incredible pierogi. Some people eat pierogi as a main course, but I prefer them as a hearty appetizer. I add goat cheese to the potato filling for extra creaminess, and a dash of truffle oil to the creme fraiche for a luxe finish. My mom says this is her favorite dish of all my creations, and that's saying something.
To make the dough: Whisk the flour, baking powder, and salt together in the bowl of a heavy-duty standing electric mixer (or in a large bowl). Whisk the sour cream, water, eggs, and oil together in a small bowl, then pour into the flour mixture. Using the paddle attachment, mix on low speed (or stir with a wooden spoon), adding more water if the dough is too dry, to make a soft dough. Change to the dough hook and mix on medium-low speed until the dough is smooth and supple, 6 to 8 minutes. (Or turn out the dough onto a lightly floured surface and knead by hand for 8 to 10 minutes.) Wrap the dough in plastic wrap and let stand at room temperature for 1 to 2 hours.
Meanwhile, make the filling: Put the potatoes in a large saucepan and add enough cold salted water to cover by 1 inch. Bring to a boil over high heat. Reduce the heat to medium and cook until the potatoes are tender, about 20 minutes. Drain well. Press the potatoes through a potato ricer (or rub them through a coarse wire sieve) into a medium bowl and let them cool.
Heat the olive oil in a small skillet over medium heat. Add the red onion and cook, stirring occasionally, until tender but not browned, about 3 minutes. Stir the onion into the potatoes, along with the goat cheese and chives. Stir in the cream and season to taste with salt and pepper.
To assemble the pierogi: Line a large rimmed baking sheet with parchment paper. Working with one half of the dough at a time, roll it out on a lightly floured work surface until about 1/8 inch thick. Using a 3-inch round cookie cutter, cut out rounds of the dough, reserving the trimmings.
Spoon about 1 teaspoon filling on the bottom half of each round. Brush the edge of each round with beaten egg, fold in half to enclose the filling, and seal closed with a fork. Transfer to the baking sheet. Knead the dough scraps together until smooth and let rest for about 10 minutes. Then repeat with the remaining dough and filling. (The pierogi can be covered with plastic wrap and refrigerated for up to 8 hours.)
To caramelize the onion: Melt the butter and oil in a large saucepan over medium heat. Add the yellow onion and cook, stirring occasionally, until very tender and caramelized, about 25 minutes. Stir in the thyme and season to taste with the salt and pepper. Let cool. Coarsely chop the onions and transfer to a small bowl. (The onions can be covered and refrigerated for up to 8 hours. Bring to room temperature before using.)
To make the truffle creme fraiche: Mix the creme fraiche and truffle oil in a small bowl. (The creme fraiche can be covered and refrigerated for up to 8 hours. Bring to room temperature before using.)
Position a rack in the center of the oven and preheat the oven to 200 degrees F. Line another large rimmed baking sheet with paper towels.
To cook the pierogi: Heat the oil in a large skillet, preferably nonstick, over medium heat. In batches, without crowding, add the pierogi, flat sides down, and cook, turning once, until golden brown on both sides, 4 to 5 minutes. Using a slotted spoon, transfer the pierogi to the baking sheet and keep warm in the oven while cooking the remaining pierogi, adding more oil to the skillet as needed.
Arrange the pierogi on a platter. Top each with a dab of the caramelized onions and sprinkle with the chives, if using. Serve immediately, with the truffle creme fraiche.