Makes 20 pierogies Ingredients For the dough (should be made at least an hour in advance) 3 cups flour 1 teaspoon salt 2 tablespoons sour cream 1 egg, beaten ¾ cup water For the filling 3 russet potatoes, peeled, boiled and mashed ¾ cup cheddar cheese, shredded 2 teaspoons black pepper 1 teaspoon salt 2…