In a cast iron frying pan over, medium heat, sweat out the leeks and onions; After they start to take color add the garlic and cook until fragrant (~1 minute)

Combine the leek / onion / garlic mixture with the mashed potatoes, and stir in the cheese

Take one dough disc and place one tablespoon of filling in the center, fold over and crimp with a fork; repeat until all disks are used

In a large pot, bring 1 gallon of water and 1 tbsp of salt to a low boil.

Add 4 pierogi's at a time and cook until they float (~3 minutes); move to a draining rack and drain; repeat until all pierogi are cooked.

In a large frying pan over medium high heat, foam out 1 tbsp of butter Add 4-6 pierogi to the pan and cook until browned and crisped

Serve with Sour Cream or Applesauce.