The Nightmare of Sigil
Native Flora and Fauna
Sigil does in fact have its own native flora and fauna. A limited, unpleasant population for sure, but provides a confirmed safe to consume source of food.
Oozes, the primary and possibly only fauna within the city, are quite abundant. Multiple types of oozes have been found (with some forms explicitly warned to be avoided). Oozes as a food source are used as 'food extenders', for their strange ability to take on the flavor of food they're cooked with. On its own, ooze is fairly tasteless, but satisfying and filling.
Fungus, the primary fauna within the city, needs to be selected with scrutiny, however there are plenty of fungal types to be found that actually have pleasant flavors and aromas. Fungus is more known for its side effects though, that many of the types found have substances that can alter the mind, the vision, and in general be more like alcohol than a food.