kohlrabi
The swollen, bulb-like stem of a cabbage plant, pale green or purple, with a mild, slightly sweet cabbage flavour.
In German-speaking countries kohlrabi is in almost every supermarket; in English-speaking countries look for it at farmers' markets, well-stocked produce sections, Eastern European or Asian grocers, and vegetable box schemes. Small, young bulbs are more tender, while large ones tend to turn woody.
Substitute:
Peeled broccoli stems come closest in flavour and bite; young turnips also work but taste sharper and soften more when cooked.