Brussels sprouts—the variety of cabbage with a stout stem and a budlike head of tightly folded leaves—are named after the city of Brussels. The modern version of the plant was originally grown primarily in the area of Northern Europe now known as Belgium, of which Brussels is the capital. In the name of the cabbage, Brussels is usually capitalized like the name of the city, though some publications don’t capitalize it.
Brussel sprouts—without the s in Brussels—is a common misspelling—for example:
We went ahead and ordered some fried brussel sprouts, housemade pimento cheese, and a Twinkie milkshake. [Charleston City Paper]
A stunningly prepared pistachio crusted halibut with masala butter sauce, arancini (fried risotto ball) and shredded brussel sprouts. [Edmonton Journal]
And these writers spell Brussels sprout(s) correctly:
Vanity Fair is reporting that Prince William and Catherine, the Duke and Duchess of Cambridge, ate Brussels sprouts during their honeymoon is Seychelles. [Washington Post]
The problem with food experts is that they have never needed to shift so much as a brussels sprout from a shelf in their lives. [Daily Mail]
These methods work with multiple types of cabbage, and can also be used to shred Brussels sprouts. [In The Kitch]