Potato and Cheese Pierogy Filling

Why I Love This Recipe
My mother married twice. The second husband was a Ukrainian and she made these often. This looks complicated but it is not. Just time consumming.
Ingredients You'll Need
4 white potatoes
1 cup of Cheddar cheese, cubed or shredded
1/2 chopped onions (sauted)
salt and pepper to taste
Directions
Bring 4 cut potatoes to a boil.
Drain potatoes and reserve potato water for pierogy dough.
Mash the potatoes
Add cheese, onion, salt and pepper to the potatoes.
Mash potatoes and cheese thoroughly until they blend into a smooth paste.
Pour filling into a bowl and cool before filling pierogies.