Romanesco cauliflower is an heirloom -- not a hybrid of cauliflower and broccoli, or cauliflower and an M.C. Escher print -- and also a near-perfect example of naturally occurring fractal. (Fractal: A fragmented geometric shape composed of smaller parts that are copies of the whole.) Mathematicians actually study them, hopefully before eating them for dinner.
As it seems a shame to puree something so utterly beautiful for soup, we'd suggest roasting them instead. Some olive oil, sea salt and black pepper maybe, then into a very hot oven. If you want something slightly fancier, here's a Deborah Madison recipe for cauliflower and Brussels sprouts salad with mustard-caper butter. Or just string some cranberries around the whorls and worry about eating them later.