In a large bowl, combine 3 cups all purpose flour and 1 teaspoon salt.

Add 1 large egg, ¾ cup sour cream, and ¼ cup melted unsalted butter. Mix until a soft dough forms.

Turn the dough out onto a lightly floured surface and knead for 5 to 7 minutes until smooth. Cover and let it rest for 20 minutes.

In a bowl, mix together 2 cups mashed potatoes, 1 cup shredded cheddar cheese, ½ teaspoon salt, and ½ teaspoon black pepper until well combined.

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

Cut circles using a 3 inch cutter or glass.

Place about 1 tablespoon of the potato and cheese filling in the center of each circle.

Bring a large pot of salted water to a boil.

Add the pierogi in batches and cook for 3 to 4 minutes, until they float to the surface. Remove with a slotted spoon.

Heat 2 tablespoons butter in a large skillet over medium heat. Add the chopped onion and cook for 5 to 7 minutes until golden.

Add the boiled pierogi to the skillet and cook for 3 to 4 minutes per side until lightly browned.

Serve warm with the sautéed onions.