1. Some people use egg for the dough. I tried all kinds of dough variations and in my opinion, mine is the most delicate. Egg tends to make the dough tougher. You can add an egg to the dough and reduce the amount of liquid.
2. Addition of blue cheese and cheddar is something new to me. It's a simple trick from a very famous Polish chef. I tried it and it really elevates this dish to another level.
3. If you are not serving pierogi immediately, take them out on a counter and let them cool down and dry a little bit. Make sure that they are not touching each other. Once cooled, place them on a baking sheet, again with distance from each other, place in a freezer and let them freeze. Once done, you can pack them in the plastic bag.