I ordered the FBB Mushroom (Zł 19) with 200gr patty, roasted mushrooms, smoked cheese, caramelized onions, and mushroom mayo. This was everything I always wanted when ordering something that had mushroom in their name, it was incredibly intensely mushroom flavored, and the onions and cheese balanced and complimented it perfectly. The meat and the bread are fantastic quality, super fresh, and tasty.
The real problem, though, was how long it took us to get our food; one hour after we ordered. They seemed to be a bit understaffed, so I would only recommend going there on a quieter moment (definitely not a Friday night). But overall, the food was excellent quality and quite tasty, we rate it an 8/10.