This is the first essential ingredient: boerenkool, or kale in English. In case you are not quite sure of what kale is, I have already looked it up for you in Wikipedia.
Kale is a form of cabbage (Brassica oleracea Acephala Group) in which the central leaves do not form a head. It is considered to be closer to wild cabbage than most domesticated forms. The species Brassica oleracea contains a wide array of vegetables, including broccoli, cauliflower, and Brussels sprouts. The Cultivar Group Acephala also includes spring greens and collard greens, which are extremely similar genetically.
Kale is good source of iron, vitamin C and calcium. Due to its high nutritional value it is often recommended by vegetarians as a way to consume many good nutrients.
** To prepare it for this recipe, just chop it into smallish strips. Or buy pre-chopped if you live in Holland!