Using a food processor or by hand, mix sour cream, flour and salt together. Knead dough until smooth and not sticky. Let rest. For the filling: Boil potatoes until soft; drain. Mash until smooth. Fry onion in margarine until softened. Add warm mashed potatoes to onion/margarine mixture. Season with salt and pepper and add shredded cheddar cheese or if you prefer, add velveeta or sliced processed cheese. To shape: Roll dough about 1/4 inch thick on very lightly floured board. Cut 2 inch circles from the dough. Add a large tablespoon of potato/cheese/onion mixture onto one half of the circle. Fold over and crimp edges well so that it is the shape of a "pizza pop." I find it easier to work with if I turn over the dough circle and put the potato mixture on the floured side of the dough. Reroll the scraps of dough. It rolls easily and is easily reshaped. To cook: Boil about 2 quarts of salted water. Throw in about 12 perogies and let cook until they rise to the top of the water- only a few minutes. Drain and continue cooking the rest of the perogies. Notes: 1. Depending on the size of the potatoes, you can have too much dough or too much potato mixture or be just right. The dough is easily made so you can make more up quickly if you need to. 2. Extra dough or potato mixture can be refrigerated for a couple of days. 3. If you like, you can freeze the perogies on a lined cookie sheet after boiling. Bag them when frozen. 4. My favourite way to prepare the perogies after the boiling or the defrosting is to fry them until golden brown with onions. Serve with sour cream. Recipe by: Carole Walberg Posted to JEWISH-FOOD digest by Bob & Carole Walberg on Oct 20, 1998,