"2 zrazy stuffed WITH mashed potatoes, stewed cabbage, mushrooms, onions and carrots, AND served with sour cream"
Since you are introducing us to a new word, you can make the plural of zrazy be as you like: "zrazy" or "zrazies". I personally think "zrazy" sounds better because it sounds less like English, and that's actually a good thing for a foreign dish.
The potatoes are part of the stuffing, right?