Saute onion in butter until just soft. Add kapusta and brown. Season with salt and pepper to taste. Allow time for the filling to cool before assembling pierogi, otherwise they will fall apart when cooked. If making more than one filling, this one can cool while you prepare the others.
To assemble and cook pierogi: Roll out a portion (or half portion, if space is limited) of the dough on a floured surface as thinly as possible -- somewhere between a wonton and a thin pizza. Put about a tablespoon of filling on the dough and use a large plastic cup (a big gulp-type cup works really well) to cut a circle around the filling. Dampen half of the circumference of the circle with a bit of water on your finger and fold into a dumpling. Boil the dumplings a dozen at a time in a big pot of salted water until they float -- no more than 3 minutes. At this point you can freeze them in bags for later use. To prepare for serving, brown the pierogi in a skillet using a bit of butter. If you want to be really traditional you can use bacon grease. Alternately, you can bake them in a casserole, spraying both the casserole and the pierogi with a butter flavored spray, and putting a few pats of butter on top of the dumplings.
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