In mixer with dough hook place Sifted Flour, Baking Powder, Baking Soda, Warm Water, Sour Cream, Eggs and Melted Butter and run until dough comes away from sides.
Let dough rest 20 minutes before rolling and cutting into circles. Roll out fairly thin and with biscuit cutter cut into 4-inch circles. Stack dough circles for filling. Have a bowl of water and pastry brush on hand to lightly dampen edges of dough seal.
Filling
Sauté Onions until soft.
In mixing bowl place Mashed Potatoes, Sautéed Onions, Sauerkraut and Shredded Cheddar Cheese and blend well.
Assemble & Cook
Take a circle of dough and brush edges with water. Place a spoonful of filling in center and fold dough in half. Press edges to close and lightly crimp with fork to secure.
Place pierogies in large pot of lightly salted water and remove when they float to the top. Serve hot with a little melted butter or place in skillet with a little butter and a drop of olive oil and sauté until lightly brown. Serve with grilled or roasted kielbasa and sauerkraut with bacon.