It's been said before, and I'll reiterate because melted lard/butter and bacon bits with sautee'd onion are the best way to go. No better way, this is how my grandmothers (both Polish) made them, and this is the way I will continue to eat them.
UNLESSSSSS- you have made your pierogi with fruit inside, not meat or potato. Try it if you haven't- make the same exact dough, and just put in cherries, or slices of apple, or raspberries, pretty much anything you want. Then eat them either just by themselves, or with a bit of sugar. Definitely a good desert.