In the image to the right you see three of the most popular fillings made at home. Kasa and cheese, mushroom and potato and cheese.
In this page is a recipe for the kasa and cheese filling and a video that shows how it is made.
When you add this filling to a pierogi dough made with spelt flour, you have a rather healthy pierogi.
The recipe for and a video showing how to make a spelt pierogi dough is at Spelt Pierogi Dough.
Kasa is very popular in Poland. And there are often leftovers. This filling allows for the use of the leftovers to make a pierogi filling.
Of course you can prepare the filling with a freshly boiled batch of kasa. But if there are leftovers around, this is the place to use them.
The recipe is simple. Equal parts of kasa, onions and white cheese. Then add salt and pepper to taste.
The final mix should be stiff. It must be that way for you to properly fill the pierogi. See How To Fill Pierogi.
A variation of this filling is to substitute meat for the cheese.
When mixing with meat, either boil the kasa in broth or bullion or add some when you combine the two.
Information about meat fillings is at Meat Filled Pierogi.