To make the wrappers, in a small bowl, combine the egg yolk, milk, 1/2 cup water, and the vegetable oil. Whip with a fork 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 to sticky, add a little more flour, about 1 teaspoon 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 into a small bowl, cover with plastic wrap, and refrigerate for 20 minutes. Clean and dry your work surface.

While the dough is chilling, prepare the topping and filling. Melt butter in a large sauté pan, then add the onions and sauté over medium high heat, stiring occasionally, until browned, about 10 minutes.

In a large bowl, combine 3/4 cup cooked onions, the mashed potatoes, the farmer's cheese, salt and pepper. Set aside at room temperature. Reserve the remaining onions for the topping.

When you are ready to roll the dough, in a small bowl combine 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 the measuring spoon). Set aside a floured jelly-roll pan, platter or cutting board for finished pierogi as well.

Divide the dough into 3 sections. Place 1section on the work station, 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, leavibg an empty margin. Brush some of the egg white mixture on half of the outer edge of the circle, and fold the dough over into a half moon shape. Crimp the edges with your fingers or with a small fork.

As you finish, trasfer each pierogi to a floured board or platter. Do not stack them. Repeat with remaining dough and filling.

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 yhe boiling water and cook for 4 minutes. Remove with a slotted spoon, drain, and transfer to a serving platter. Repeat with the remaning pierogi, allowing the water to return to a full boil each time.

When you have cooked all the perogi, serve them topped with the reserve sautéed onion and as much sour cream as you like.