Mix first three ingredients until well-blended. Set aside.
_Filling_
Mash potatoes. Cook chopped onions in butter. Mix.
Roll dough on flour. Cut circles in dough (approximately 4" in diameter). Fill with 1 tbsp filling each. Fold over and seal with fork. Boil gently until pierogies float to top. Lift with straining spoon. Fry until light brown.