Actually, you can fill them with almost anything, maybe you just didn't like the filling you were offered back then. Here are a few options:
Pierogi z tartymi ziemniakami, boczkiem i cebulą - Pierogi with mashed potatoes, bacon and onion.
Pierogi z (kiszoną) kapustą i grzybami - Pierogi with (sour) cabbage and mushrooms.
Pierogi z (białym) serem - Pierogi with curd.
Pierogi z łososiem i szpinakiem - Pierogi with salmon and spinach.
This is interesting actually. The english words "very" and "truly" are both derived from the words for "true" in Latin and Old English respectively - "verus" and "treowe". It's a result of the Roman empire that these words really are interchangeable in English for this sentence.
Really and very are synonyms (not every use, of course, but overlap). If I said, "This is really hot," it means that it is very hot. If I wanted to use the other definition, I would say, "This is actually hot." In fact, I think it is more common to say "really" in a sentence like this. http://www.thefreedictionary.com/really