The reason your pierogies stuck is because they were wet and because you tried to move them before the bottoms were fully crisped and released from the pan surface.
Forming a bond with a smooth surface is something food with a lot of starch or protein on the surface does. And, moisture makes it worse. You created quite a hurdle for yourself by steaming the pierogies first. The best method for dealing with frozen piergoies is either to fry or boil, and not steam or boil first.
You can cook them frozen or defrosted. If frying you'll have better luck with defrosted dumplings. But ... you CAN cook them frozen. You can even boil or steam them -- but you've got to do more than partially dry them. You'll have to dust them with flour or other starch to get a completely dry surface.
To fry, start by preheating the pan at medium high heat. Then add either a tsp or so oil and a 2 or 3 tbs of butter, or about 2 tbs of oil (for a 10" pan). Reduce the heat to medium low. When the butter foam subsides, or the oil is heated add the pierogies to the pan without crowding them. The oil in the pan will immediately cool and the pierogies will stick. Allow the bottoms to fully cook before shaking the pan, four or five minutes. The pierogies should release by themselves, so you can easily turn them.
If some do not, turn those that do and allow the stickers some more time. After another minute try shaking the pan again. If some still don't release, try another 30 seconds and some more shaking. Next, try tapping the stickers on the side with your spatula, rather than digging underneath them. Finally, do your best to dig under the the stickers with your thinnest, sharpest spatula -- preferably a slotted, flexible, fish turner.
Remove, drain and serve with melted butter seasoned with dill, marjoram or tarragon, and/or sour cream. If you fried with butter, you can season the butter while the pierogies drain, and serve them in the cooking butter.
Except for the low heat used for the dumpling skins here, this general technique of waiting for the food to release -- rather than trying to turn it early -- is typical of most hot pan technique -- whether saute or sear. Waiting for the release is one of the most important techniques any cook can learn. Even more important, and difficult for many people to assimilate, is the larger idea of allowing the food to cook to a given point without interfering. It's one small part of "respecting the ingredients" chefs talk so much about.
I disagree with KissTC to the extent that dumplings may be pan fried in oil and often are. However, I don't quite get the whole pierogies in olive oil thing. Doesn't seem like it would be a good combination at first blush. But I wasn't there to taste. How were they?
A different method of cooking pierogies is simply to steam or boil them and forget frying them altogether. Serve in a little melted butter with sour cream on the side.
Hope this helps,
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