Mix the flour and salt, place into a bowl and make a well in the centre.
Crack the eggs into the flour. Using a fork, stir the eggs into the flour.
Slowly add the water while kneading, until the dough forms a sticky ball.
Continue to knead the dough on a floured surface until dough is soft but not sticky. Knead the dough for about 5 minutes.
Cover the dough with a bowl and allow to rest for about an hour. If you're in a hurry you can use after 30 minutes of rest, but an hour is best.
In the meantime, bring potatoes to a boil and cook until soft (about 15 minutes). Drain, allow to cool, mash until smooth and set aside.
Finely dice the onion. Fry in a frying pan with butter until caramelized. Add to the mashed potatoes.
Add the grated cheddar to the mashed potato mixture and mix it all together.
Roll out the dough about ⅛" thin on a lightly floured surface. Using a 3" cookie cutter, cut out circles. Remix, roll out and reuse the extra dough to make more circles. Cover the dough that you aren't using so it doesn't dry out.
Place about 2 teaspoons of potato and cheese filling in the centre of the circe. Carefully fold in half and pinch to seal the perogy shut. Careful not to have the filling touching the edges, or it will fall apart when cooking.
Place the perogies on a tray and freeze for at least 30 minutes. Place the individually frozen perogies into a Ziplock bag for an easy dinner later.
Bring a large pot of salted water to a boil. Place the perogies into the water and boil until they float to the top, about 5-7 minutes. Frozen perogies will take longer.
Serve tossed with butter, sour cream and caramelized onions.