My most favourite pierogi filling is what's called in Poland "ruskie". Ruskie pierogi arrived from a prewar region of Poland which is now part of Ukraine. Indeed, before 1945 Ukrainians used to call this particular variety of pierogi "Polish pierogi". It is likely that “pierogi ruskie” were created by Poles living in Ukraine at the time.
It's a simple filling of quark cheese, mashed potatoes and caramelized onion. As I cut out dairy from my diet I substitute it with firm tofu - works perfectly! It's equal parts of crumbled tofu to mashed potatoes, one large onion, sliced and caramelized on a bit of oil, all mixed together, seasoned well and your filling is ready.