Mix the flour and salt in a bowl. Add the egg, sour cream, and melted butter. Stir until a soft dough forms. It should feel slightly sticky but manageable.

Turn the dough onto a floured surface. Knead it gently for five minutes. Wrap it in plastic wrap. Let it rest for 30 minutes at room temperature to soften.

Mix your warm mashed potatoes with the shredded cheddar cheese. Add salt, pepper, and minced onions. Stir until the cheese melts and the mixture is smooth. Let the filling cool down completely.

Unwrap the dough on a floured table. Use a rolling pin to make it very thin. It should be about 1/8 inch thick. Make sure it does not stick to the table.

Use a 3-inch round cutter to make circles. Space them closely to get as many as possible. Remove the extra dough scraps. You can re-roll the scraps one more time later.

Place one tablespoon of potato filling in the center of each circle. Do not overfill them. If you add too much, they might pop open while cooking in the water.

Fold the dough over the filling to make a half-moon shape. Pinch the edges firmly with your fingers. Use a fork to press the edges for a pretty, secure seal.

Bring a large pot of salted water to a boil. Drop the pierogies in small batches. Do not crowd the pot. They need room to move around as they cook.

The pierogies will sink at first. Wait for them to float to the surface. Let them cook for 2 more minutes once they float. Remove them with a slotted spoon.

Melt butter in a large skillet over medium heat. Place the boiled pierogies in the pan. Cook until both sides are golden brown and slightly crispy. This adds great flavor.

Plate your hot pierogies immediately. Top them with a big dollop of sour cream. Sprinkle fresh chives over the top. Enjoy your homemade masterpiece while it is still warm!