Scrub potatoes clean under cold water and prick deeply with a fork or knife. Microwave potatoes until completely soft (about 9 minutes for me). You could also bake them in your oven or use some left over baked potatoes.
Cut cooked potatoes in half and scoop out the insides into a large bowl. Mash potato with butter, sour cream, salt and pepper. Stir in milk to get a smooth consistency. Stir in cheese. Set aside. **You could also use left over mashed potatoes! Just be sure to stir in the cheese and more milk or cream to get a smooth consistency**
In small bowl, whisk egg with water. To assemble pierogies, take one wonton wrapper and brush the edges with the egg. Place 2-3 teaspoons worth of potato filling into the center. Fold wrapper in half to create a half moon shape and press edges together keeping the potato filling inside and removing any excess air. Repeat this process until no more filling remains. Depending on how much or little you fill your pierogies, you could end up with more or less than my 36 I got.
Place pierogies on baking sheet in a single layer. Cover with plastic wrap and freeze until solid, about 2 hours. Transfer to freezer bags until ready to cook.
To cook, bring a large pot of salted water to boil. Meanwhile, in a large skillet, start rendering the chopped bacon. After about 3 minutes of cooking the bacon, add in chopped onion. Once water has come to boil, drop in pierogies and stir. After 2-3 minutes, pierogies should be finished cooking and should float to the top. Using a slotted spoon, drain pierogies and drop into skillet with bacon and onions. Fry pierogies until golden brown on either side or until cooked to your liking. Bacon and onions should be completely finished cooking by this point. Serve warm with sour cream, more cheddar cheese and/or chopped green onions.