Cabbages are a low-tier food grown from Seeds found in the Erebus. To acquire Cabbage Seeds, simply break any type of Long Grass-like plant (including vanilla Long Grass found in the Erebus) until you find some.
Cabbages, as mentioned above, are a low-tier food; they only satisfy 1 hunger point (0.5 hunger icons), and make the player fart. Yes, you read that right.
HOWEVER, Cabbages are very useful for the creation of Titan Stew. They have no other purpose at this time.