The u Fukiera restaurant at the heart of the old town is a long established superb restaurant serving Polish (the Greater Poland considered, as some dishes are now native to Poland's neighbours), run by one of Poland's best known restaurateurs. She has a number of different restaurants in Warsaw.
I am not sure what her trick is, but everything she cooks is fantastic. The zurek (sour soup with sausage and egg) is matchless, and the incredible barszcz (beetroot soup) is even better. The schabowy (breaded pork cutlet) is giant and served with surprising (oops, it will not be anymore) sour cabbage.
Rynek Starego Miasta 27 Warszawa Mazowieckie Poland