Zapiekanka is definitely the most popular street food in Poland. It is stale bread with melted cheese and ketchup. Often, there's another topping involved. Ours had mushrooms. Some have meat. Fancier ones may be called "hawaiian" and include canned pineapple or "greek" and include olives. While a lot of street vendor signs also read "hamburgy" and "hot dogy" (which I think translates into hamburger and hot dog, though my Polish isn't great) the only thing you really see people walking around with is zapiekanka. I'm surprised there isn't some Polish-American kid making a killing selling this stuff out of their dorm room. All one would have to do is invest in a grater and buy a block of mozzarella cheese now and then. Then, you swing buy a bakery after hours and snatch the bag of unsold baguettes left out on the curb. Save the ketchup packets that come with just about any order of takeout and voila! you've got yourself a zapiekanka business.
Our zapiekanka was purchased at this roadside food van and was probably not the best, but on a drive that didn't offer any other food options, we figured it was the perfect opportunity to try it out. Most of the zapiekanka we had seen around Wroclaw and Krakow was much, much smaller and crispy, toasted. Ours was limp and soggy, the result of a quick microwave warm-up by the friendly vendor, and had to be held up with two hands. It was delicious nonetheless (especially with the mushroom surprise) and went down far too easily. It reminded me of the Elios pizza squares I used to get at my town pool's snack bar. Only the size of Merlin's forearm.