Start preparing the filling. Fill a large pot with cold water and put it on the stove, then salt the water generously. Get the Yukon Gold Potato (10) into the pot and bring the water to a boil. Boil the potatoes until they are completely tender and mashable, which takes about 20-25 minutes.
In the meantime, make the dough so it can rest. In a bowl combine the All-Purpose Flour (2 cup), Sour Cream (8 ounce), Egg (1), and Salt (1/2 teaspoon). Stir it together completely and then turn it out onto a lightly floured surface. Knead it for a couple of minutes to have it come together in a wonderful ball of dough. Wrap it in plastic and set it aside to rest for about 30 minutes.
When the potatoes are done, it's time to make the filling. Remove them from the water with a slotted spoon and transfer them to a very large bowl, but do not drain the water. leaving the pot of water on the stove boiling.
Mash the potatoes in the bowl and stir in the Farmers Cheese (1 carton), American Cheese Slice (6 slice), and Butter (1 tablespoon) until it becomes a creamy, luscious filling. Add Salt (1 pinch) to taste, a big pinch should do it. Set the filling aside.
When the dough is done resting, divide it in half. Roll out the first half to about 1/4 inch thick on a floured surface with a floured rolling pin. With a 4 inch round cutter, cut out perfect little circles. You should get nine circles the first time.
Take the scraps and roll them out again to cut out another 3. Repeat this process with the other half of dough to get 24 circles in total.
Fill each of the circles with two tablespoons of filling on one half, then bring the other half over to enclose it and pinch it together thoroughly. Really seal them even more and make a pretty pattern along the edge by pressing down with the tines of a fork over the whole edge.
Gently drop the pierogi into the pot of boiling water you used for the potatoes. That extra starchiness just makes them so good! They only need to boil for a minute. Remove them with a slotted spoon to a platter. Then it's time to finish them up!
Heat the Butter (1 tablespoon) in a large cast iron skillet. Add the Onion (1) and let it get soft and fragrant for a minute. Gently toss the pierogi in the pan with the onion to let them get slightly golden. Then plate them and serve them immediately! Serve with kielbasa sausage to have a perfect Polish feast.