My most favourite pierogi filling is what's called in Poland "ruskie". Ruskie pierogi arrived from a pre-war region of Poland, which is now a 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.