In a large mixing bowl, combine the flour and salt. Create a well in the center, add the egg and water, then mix until a dough forms.

Transfer the dough to a floured surface and knead for about 5 minutes until smooth.

Using a rolling pin, roll the dough to about 1/8 inch thickness.

Use a round cutter or glass to cut out circles from the rolled dough.

In a bowl, mix the mashed potatoes and cheese until well combined.

Place a small spoonful of filling onto each dough circle. Fold the dough over and pinch the edges tightly to seal.

In a skillet, cook the sausage over medium heat until browned and cooked through.

Boil the filled pierogies in salted water until they float, about 3-4 minutes. Drain and fry in melted butter until golden brown.