Make the dough: In a large bowl, whisk together flour and salt. Stir in beaten egg and cold water until a smooth dough forms.

Knead and rest: Turn dough onto a lightly floured surface and knead for 3–4 minutes until elastic. Cover and let rest 15 minutes.

Prepare filling: In a bowl, combine mashed potatoes, crumbled bacon, cheddar cheese, sauerkraut, salt, and pepper.

Shape perogies: Roll dough to 1/8-inch thickness. Cut 3-inch rounds. Place 1 tablespoon filling in center of each round. Fold and pinch edges to seal.

Boil: Bring a large pot of salted water to a gentle boil. Drop perogies in batches and cook 5 minutes or until they float to the surface.

Serve hot: Drain perogies and toss with butter and sautéed onions if desired. Tip: For extra crunch, pan-fry boiled perogies in butter until golden.