(Gather all ingredients.)

(Make sauerkraut filling: Melt butter in a skillet over medium heat. Cook and stir onion in hot butter until translucent, about 5 minutes. Add sauerkraut and cook, stirring occasionally, for 5 more minutes. Season with salt and pepper, then remove to a plate to cool.)

(Make potato filling: Melt butter in the same skillet over medium heat. Cook and stir onion in hot butter until translucent, about 5 minutes. Stir in mashed potatoes, salt, and white pepper. Remove from heat.)

(Make dough: Beat together sour cream and eggs in a large bowl until smooth. Sift flour, baking powder, and salt into a separate bowl; stir into sour cream mixture until dough comes together.)

(Knead dough on a lightly floured surface until firm and smooth.)

(Divide dough in half, then roll out one half to 1/8-inch thickness. Cut into 3-inch rounds using a biscuit cutter. Place a small spoonful of potato filling into the center of each round.)

(Moisten edges with water, fold over, and press with a fork to seal. Roll and fill remaining dough half with sauerkraut filling.)

(Bring a large pot of lightly salted water to a boil. Add pierogies in batches to boiling water and cook until they float to the top, 3 to 5 minutes. Remove with a slotted spoon.)

(Enjoy!)