You’ll need a blender and a very large pot for which you have with a lid to make this dish. If you buy ready-sliced cavolo nero or kale, the stems will most likely be tender; if buying them whole, however, or if the stems are thicker than 1cm, you’ll need to strip off the leaves from the kale (do this simply by holding on tight and running your hand up the stem) and slice them by hand. Either compost the stems or freeze them to add to veg stock or soups