You can use any mashed potato filling you'd like. I prefer my cheddar onion mashed potatoes in mine. This recipe will make approximately 75 pierogi. Here is the recipe...
6 C flour
1 1/2 C sour cream
3/4 C water (add more if the dough seems dry)
Add all ingredients except water into a kitchen aide mixer with the dough hook. Set on stir to knead dough. Once combined, slowly add water until mixed. If it seems dry, add more water. If it is too wet, add more flour.
If you don't have a mixer, you can place the flour on a counter and add the wet ingredients in the center. Knead with hands until mixed.
Let dough stand for 20 minutes. Prepare potato filling.
Using a pasta machine or a rolling pin, roll out dough into a flat sheet approx 1/8" thick.
Cut out circles of dough.
Spoon a teaspoon of potato filling into the center of the circle. Working with your hands, gently pull at the circle to make it large enough to enclose the potato filling. Wet the edge of half of the circle by wetting your finger in the water bowl and running it around the edge of the circle. Squeeze closed.
If you're making them to freeze: Place layers of pierogi between wax sheets. Be sure to place a wax paper sheet on the bottom of the pan or they will stick. Only place three layers of pierogi in the dish and freeze for 2 hours. Place pierogi in gallon freezer bags. When ready to eat, place in boiling water for 3-5 minutes. Pierogi are ready when they float.
Making them immediately: Place pierogi in boiling water and remove when they start to float. Place cooked pierogi into a dish with melted butter & sliced onion along the bottom and between pierogi to avoid sticking together. Again, I wouldn't stack more than three layers in the baking dish.
I recommend freezing and making them fresh if you're making them ahead. You can warm cooked pierogi in the microwave, but freshly boiled is best.
Enjoy with sour cream... or hot sauce, like my husband.