To make the filling, melt the butter in a frying pan and add the onions. Cook on medium low for about 10 minutes. Pour the butter mixture into a bowl with the cooked potatoes and mash them all together. Add the cheese and stir to mix.

Cook the sliced onion in a pan with some butter on medium heat for about a half hour/45 minutes until browned.

Roll out the dough and cut 16 burger sized rounds from it. Place a scoop of the potato mixture on half of the rounds. Place the other half on top of the first half to form the pierogi. Press the potato mixture to a uniform size and shape, and make sure to get most of the air out. Seal the pierogi on the edges with a fork.

Bring water to a boil and cook the pierogi until they float, about 3-5 minutes. Remove from water and add to a frying pan with butter. Sear on both sides to golden brown.

Meanwhile, form burger patties the same size as the pierogi.

Once all the pierogi are done cooking, add the burgers to the pan and cook on high heat about 4 minutes per side to a medium doneness. Add cheese after flipping.

Put a pierogi on a plate and place the burger on top. Add the onions and some sour cream and top with another pirogi. Serve!