To make the dough

• Heat leftover mashed potatoes and milk until just warm. You can do this on the stove top or in the microwave. Your choice.
• Add flour and salt to potato / milk mixture.
• Stir with a fork until combined, then turn the contents of the bowl out onto the counter and knead until a slightly sticky dough forms.
• Place in an oiled bowl. Cover and let sit for 30 minutes.

To make the filling

• Melt 1/4 cup of butter in a small pan over medium heat.
• Add onion, lower the heat to medium low and cook until onions are browned and soft.
• Add 2 cups of mashed potatoes and stir to combine. Now add 2 cups of shredded cheddar cheese. Set aside.

Assemble the pierogis

• Roll the dough as thin as possible on a floured board.
• Cut out 3" circles of dough using a cup or biscuit cutter.
• Add 1 level teaspoon of filling to each circle.
• Pinch all around the dough to make sure no filling can ooze out. And continue until you're out of dough.

Now you have 2 options:

To Freeze-

• Flash freeze on a baking sheet for 1 hour. Place in a freezer bag and store in freezer. To cook from frozen add frozen pierogi to boiling water and cook as directed below.

To Cook-

• Place 6-8 pierogi in boiling water and gently stir once. When pierogi float to the top set a timer for 1 minute. Remove pierogi with a slotted spoon.
• Pan fry in melted butter until crispy.
• Serve with sauted cabbage cooked with kiebasa!
• Delicious.