Combine. Set mashed potatoes and cheese aside. Add flour gradually to the last 3 ingredients. Mix until dough rolls firmly. Divide dough into 4 piles. Roll thinly and cut 1" squares. Fill with small amount of potatoes and cheese mixture. Pinch together tightly. Place in boiling water, boil until perogies float. Remove and place in frying pan with 3 tablespoons oil and small onion. Toss perogies lightly until coated. Perogies can be cooked longer for those who like them brown and crunchy.