To make Perogies: Combine filling together with food processor. Set aside. Cut dough with a 3.5 inch circular cutter. Put 1 1/2 teaspoons potato filling in each circle. Fold in half and seal edges. Lay on parchment paper lined baking sheet. To Cook: In a kettle, bring 4 quarts water and 3 teaspoons salt to a boil. Add pierogies and cook for 4-5 minutes. Drain. Fry with butter and onions if desired.