A Fennel Bulb, the nicest part of the Fennel Plant
It's just onion. With flowers.
It grows all over the Galaxy, even on Earth, which has its own seperate flora and fauna from most other planets reseeded by the Forerunners, which tend to have very similar vegetation.
Fennel can be eaten too, and it is served in all good resteraunts, even Grunt Stop. In fact, Sood is known to eat Fennel, and nothing but Fennel. When Mr. Kreutzfeld entered Sood's office, he snapped all the Fennel sticks in the little bowl Sood keeps by his computer, claiming to be 'just checking they weren't Objects'.
Fennel can also be stewed, roasted, poached, scrambled, fried and minced. It is also used as a substitute for lolly ices, 'Fennel-On-A-Stick' is a popular snack for kids across the Galaxy. Even Elites and Brutes will lay down their arms and make friends if someone brings Fennel to the fight. This led to Johnny starting an advertisement campaign for Fennel, saying 'Fennel the Fight'.
Even Prophet of Truth and Lord Hood settle their differences and eat Fennel at the Grunt Stop every now and again, before each was killed by their respective champions. Although Hood would only ever eat Fennel with Terrantimes.