Cut the cooked potato into chunks and place in a large bowl (you can mash and mix by hand, or, do as we did, and use your mixer). Mash the potatoes with the tablespoon of butter (it will melt from the heat of the potatoes).

Cook the bacon until crispy. Set aside to drain and cool on a paper towel lined plate.

Pour about 1-2 tablespoons of the bacon grease into the potatoes. Leave 1-2 tablespoons in the pan.

Heat the pan back up and add the onions with a pinch of salt. Cook until translucent.

While the onions are cooking, crumble the bacon and add it to the potatoes.

Add the shredded cheese to the potatoes. Mix everything thoroughly.

When the onions are lightly browned, add them to the potatoes. Mix.

Taste for salt and pepper (it should not need much salt thanks to the bacon and cheese).

The filling is now ready to be used in pierogi.

If using to make potato pancakes, let the filling cool until it is handle-able. Form patties in your hands and then lightly dredge in flour. Fry on medium high heat in pre-heated oil until browned on both sides.