What is this green, bulbous, leafy vegetable?
As was mentioned, this is a kohlrabi. I felt more explanation should be given based on the fascinating nature of this plant.

Kohlrabi is one of the handful of cultivars of brassica oleracea. Others include: 


  cabbage, broccoli, cauliflower, kale, Brussels sprouts, collard greens, savoy, kohlrabi, and gai&nbsp;lan. Brassica Oleracea (Wikipedia)


Brassica oleracea has been cultivated in many different parts of the world to appear almost like entirely unique plants. However, each of the listed plants are, in fact, the same species with slightly varying traits. Kohlrabi, for example, has been bred to have lateral growth in the meristem while broccoli has been bred to have a large, flowering head.

Kohlrabi can be eaten either cooked or raw. It's often used in salads and slaws, and can be interchanged with collard greens or kale.