Gather all ingredients.

Make sauerkraut filling: Melt butter in a skillet over medium heat. Cook and stir onion in hot butter until translucent, about 300 seconds.

Add sauerkraut and cook, stirring occasionally for 300 more seconds. 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 300 seconds.

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, 300-450 seconds.