From The Veselka Cookbook
Makes 65-70 pierogi
1 large egg yolk
1 cup whole milk
1 tbsp. vegetable oil
3 1/4 cups all-purpose flour
Topping and Filling
4 tbsp. unsalted butter
5 cups finely chopped onion
4 cups mashed potatoes (leftovers are fine)
4 oz. farmer's cheese
2 tsp. salt
1/4 tsp. freshly ground black pepper
2 large egg whites
All-purpose flour, as needed
Sour cream, for serving
To make the wrappers, in a small bowl, combine the egg yolk, milk, 1/2 cup water, and the vegetable oil. Whip with a form for 1 minute. Place the flour in a large bowl. Make a well in the center and pour in the wet ingredients, about one-third at a time, using your fingers or a fork to incorporate the wet ingredients between additions.
When you have added all the wet ingredients, use your hands to fold the dough together. If it seems too sticky, add a little more flour, about 1 tsp. at a time to avoid making it too dry.
Transfer the dough to a lightly floured board and knead for 3 minutes. Again, add very small amounts of flour if the dough is too sticky to knead. When the dough is smooth and thoroughly amalgamated, form it into a ball, transfer it to a small bowl, cover with plastic wrap, and refrigerate for 20 minutes. Clean and dry your work surface.
While the dough is calling, prepare the topping and filling. Melt the butter in a large sauté pan, then add the onions and sauté over medium heat, stirring occasionally, until browned, about 10 minutes.
In a large bowl, combine 3/4 cup of the cooked onions, the mashed potatoes, farmer's cheese, salt, and pepper. Set aside at room temperature. Reserve the remaining onion for the topping.
When you are ready to roll the dough, in a small bowl combine the 2 egg whites with 2 tablespoons water and set it to the side of your work surface. You will also need a pastry brush, a rolling pin, a teaspoon (the table kind, not a measuring spoon), a form, and a round cookie cutter about 2 3/4 inches in diameter (a jar lid or juice glass with also work.) Set aside a floured jelly-roll pan, platter, or cutting board for the finished pierogi as well.
Divide the dough into 3 sections. Place 1 section on the work surface, well-floured, and roll out to 1/16 inch. Cut circles of dough with the cookie cutter. Place a heaping teaspoon of the filling in the center of each circle, leaving an empty margin. Brush some of the egg white mixture on half of the other edge of the circle, and then fold the dough over into a half-moon shape. Crimp the edges with your fingers or with small fork.
As you finish, transfer each pierogi to the floured board or platter. Do not stack them. Repeat with remaining dough and stuffing.
Fill a large stockpot about three-quarters full with salted water and bring to a rolling boil. Using a slotted spoon or skimmer, lower the pierogi, three or four at a time, into the boiling water and cook for 4 minutes. Remove with a slotted spoon, drain, and transfer to a serving platter. Repeat with the remaining pierogi, allowing the water to return to a full boil each time.
When you have cooked all the pierogi, serve them topped with the reserved sautéed onion and as much sour cream as you like.