Pierogies
I spent a little over a year living in Krakow, Poland recently and had the pleasure of eating authentic pierogies a number of times. While I cant say I ever succeeded in making my own (they fell apart, or were too doughy, and I eventually gave up as they are incredibly cheap and tasty to just buy). However, I can offer some observations.
First, while the traditional way of cooking pierogies is to simply boil them, many times I encountered a fried version that were also very tasty and a harder texture. Also, whether fried or boiled, they are always served with little pieces of fried lard (fried onions are sometimes substituted, but not as common). While sauces are also not traditional, I encountered at more "hip" pierogi places a variety of simple cream sauces. Some people use soy sauce as well (personally, I think soy sauce is fantastic with pierogies).
Probably the most traditional Polish pierogi is the Pierogi Ruskie (Russian pierogi), which is filled with mashed potatoes and a type of cream/cottage cheese. Also common are the Pierogi z ziemniakami i Grzybami (mashed potatoes and mushrooms) and Pierogi miesny (pierogi with ground beef). While I have, again, found a number of variations (ground lamb, chicken, turkey, spinach, etc) these are by far the most common. Serve with a warm beetroot soup and follow up with a shot of Zybrowka (vodka with a blade of bison grass in the bottle) and you have yourself a traditional polish meal.
Smacznego! (bon appetit!)