In a large bowl, mix the flour and salt together.

Create a well in the center of the flour and add the egg, sour cream, and softened butter.

Mix the ingredients together until a soft dough forms. Knead the dough on a floured surface for about 5 minutes until smooth.

Wrap the dough in plastic wrap and let it rest in the refrigerator for at least 30 minutes.

In a skillet, sauté the chopped onions in 2 tablespoons of butter until they are soft and golden.

In a mixing bowl, combine the mashed potatoes, sautéed onions, and shredded cheddar cheese. Season with salt and pepper to taste. Mix well until all ingredients are fully incorporated.

Roll out the dough on a floured surface to about ⅛-inch thickness.

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

Place about a tablespoon of the potato and cheese filling in the center of each circle.

Fold the dough over the filling to create a half-moon shape. Press the edges firmly with your fingers or a fork to seal.

Bring a large pot of salted water to a boil. Drop the perogies in batches into the boiling water.

Cook the perogies until they float to the top, about 3-4 minutes. Remove with a slotted spoon and drain well.

In a large skillet, melt the 4 tablespoons of butter over medium heat. Fry the boiled perogies until golden brown on both sides, about 2-3 minutes per side.

Serve the perogies hot with sour cream, and garnish with sautéed onions or bacon bits if desired.