Boil potatoes, drain, season and mash. Add sausage meat to a pan over medium high heat and break up with the back of a wooden spoon. Continue cooking until it becomes browned, crispy, and broken up into small bits, about eight to ten minutes. Let potatoes and sausage cool. Once cooled, combine cooked sausage, mashed potatoes and shredded cheddar. Set aside.
Add flour to a large mixing bowl. Make a well in the centre and add all the wet ingredients and salt and stir with a wooden spoon. Turn the raged dough onto a floured surface and knead for about five minutes. Add the dough to a large bowl and cover for 30 minutes.
Once the dough has rested, divide it in half. Roll out the dough on a floured surface to about six millimeters thick. Using a ten centimeter circular cutter stamp out circles.
Add a tablespoon of the cooled filling to the middle of one of the circles of rolled out dough.
Brush the edge of the circle with the beaten egg. Fold the dough over onto itself and using a fork, crimp the edges closed. Continue until all the Pierogies are filled.
Bring a pot of salted water to boil, and add eight Pierogies at a time. At the same time melt butter in a pan over medium high heat. Once the Pierogies start to float (about three minutes), use a slotted spoon to remove them and add them to the hot pan with the melted butter. Fry the Pierogies for a few minutes per side or until nice and golden brown.
Serve with sour cream and caramelized onions. Enjoy!