Side

Melt butter in a skillet over medium heat, add chopped onion and cook until translucent, add drained sauerkraut and cook 5 minutes, season with salt and pepper, then transfer to a plate to cool
Melt butter in a skillet over medium heat, add chopped onion and cook until translucent, stir into mashed potatoes, season with salt and white pepper
Beat together eggs and sour cream until smooth
Sift together flour, salt, and baking powder and stir into the sour cream mixture until dough comes together
Knead the dough on a lightly floured surface until firm and smooth
Divide the dough in half, roll out one half to 1/8 inch thickness, and cut 3-inch rounds
Place a small spoonful of the mashed potato filling into the center of each round, moisten edges with water, fold over, and press with a fork to seal
Repeat the assembly with the remaining dough and the sauerkraut filling
Bring a large pot of lightly salted water to a boil, add perogies, cook 3 to 5 minutes or until they float, and remove with a slotted spoon