To prepare the dough: Combine the mashed potatoes, shortening and egg yolks in a medium bowl.
Stir in the water and beat well.
In a small bowl sift together the 1 ¾ cups flour, cream of tartar and salt.
Mix into the wet ingredients until it forms a soft dough.
Add in the remaining ¼ cup of a flour - 2 tablespoons at a time until the dough no longer sticks to your hands.
The dough should be soft to touch and once no longer sticky, use the remaining flour to dust your work surface to knead lightly until it comes together in a ball.
Cover and let rest for 10-15 minutes on the counter.
While the dough is resting prepare the filling by cooking the bacon until crisp over medium heat in a cast iron or non-stick skillet.
Remove the bacon once cooked to drain excess grease on paper towels and chop finely once cool.
Reserve 2 tablespoons of excess grease in the pan and saute the onions until soft.
Stir in the sauerkraut and cook until soft and the flavors are blended together - about 5 minutes.
Add the bacon back into the pan and season with salt and pepper to taste.
To fill the pierogies, roll the dough out until it is about ⅛ to ¼ inch thick and using a 3 inch round cookie cutter cut into circles.
Fill each circle with about 1 ½ to 2 tablespoons of filling.
Using your finger tip wet half the circle and fold over to seal.
Crimp the sides with either a fork or a dumpling press.
At this point the pierogies can be frozen in a single layer on a baking sheet and cooked at a later date or bring a large pot of water to boil and cook the pierogies for 5-7 minutes or until they float to the top.
Pierogies can be served boiled or lightly pan fried with butter or olive oil after boiling.
Serve with sour cream.
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