Boil potatoes until soft, drain water from pan. Mash potatoes and add cheese and salt & pepper. Set aside to cool.

In a mixing bowl, place the flour and egg. Add 1/4 cup of water and stir all ingredients with a fork. The dough should be the consistency of pizza dough. Add more water (if necessary) until this consistency is attained.

Assemble the pierogies:
On a lightly floured surface, roll out a portion of dough to 1/8th of an inch thickness, making sure that there are no holes.
Cut dough into 3 inch squares and place approximately 1-2 Tablespoons of the filling mixture slightly off center.
Flip half of the dough over filling creating a triangle. Crimp all edges securely.
This is the important part. If you do not crimp the edges securely, they will break apart when boiled.

Gently submerge them into a large pot of boiling water. They will sink to the bottom. Once they rise to the surface of the water, they are done. Takes about 3-5 minutes.

Sauté some chopped or sliced onions in butter. When onions are soft and lightly browned, add the pierogies, stir and serve.