A cabbage is round, signifying the annual cycle of the torah readings culminating in simchas torah. When I was back in school, one of our rebbeim would frequently challenge the class to come up with “creative” inyanim as to explain some really arcane minhagim. It was actually fun and you would have been amazed at some of the rationales offered (e.g. cooked cabbage has a smell like that of the schach and décor of a very wet succah by the last of a rainy scuccos).