Pierogi recipe and filling from my grandmother

Hi,

I have been reading all the different versions for pierogi filling. My grandmother made her filling by cooking finely chopped onion in butter, then she mixed that into a batch of hot mashed potatoes, and then added sharp cheddar cheese cut into small pieces so it melted into the potates. She then mixed that well and used as the filling for her pierogi. I can't give amounts here as I never had exact measurements for the recipe- I just do it from memory and like grandma- a little here and there. I know what it tastes like, and it is good... She also used an egg in her pierogi dough. After they were assembled, they were boiled til they floated, and greased with butter and put in the fridge til dinner. Then she just fried some in butter. Anyone else make it like this???
Please help.

I didn't find the right solution from the Internet.

References:
polishforums.com/food/pierogi-recipe-filling-grandmother-1609/
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Thanks