Potato & Cheese filled Pierogies

Beat together first 5 ingredients; fold in flour, adding more if needed to form a soft dough; cover and let stand for 5 minutes in a warm place -Kneed dough on a lightly floured surface for 10 minutes OR run dough through the thickest setting of a pasta maker 5 times. -Divide dough into 4 pieces; cover with a moist towel. -Roll out each dough section to ⅛“ thick; cut out 3½” circles using a cookie cutter, cup, or can. -Beat together egg white and water; set aside. -Place 1 Tbls. of filling in the center of each circle; fold circle in half over filling; brush with egg wash, and seal edges by pressing the tines of a fork; place finished pierogies on waxed paper until all are complete. -Either freeze pierogies in a single layer, then store frozen pierogies in a sealable plastic zipper bag OR boil filled pierogis in water for 5 minutes, or until they float. To cook frozen pierogies, boil for 10-12 minutes.