Fennel flower is a common market name for nigella seed (Nigella sativa), also called black cumin, black seed, or kalonji. Despite the name, it is not related to true fennel (Foeniculum vulgare); the two plants are in different families, and the name comes only from a faint resemblance in aroma.
The small, matte-black seeds have a pungent, slightly bitter flavor often compared to a mix of onion, oregano, and black pepper. They are toasted and sprinkled over naan and other flatbreads in South Asian and Middle Eastern baking, used whole in Bengali panch phoron and other spice blends, and scattered over rye bread in Eastern European baking. Nigella seed should not be confused with true fennel pollen or fennel seed, which come from the fennel plant and taste of anise, not onion.