I steamed the purple cauliflower,
and then, as suggested, made a version of Mostly Eating's recipe for Vivid Cauliflower and White Bean Puree (the timing for which could not have been better!).
Using a few scoops of leftover white beans which had been cooked in their liquid with some sauteed leeks and fennel in place of the canned butter beans, and swapping a bit of chevre (only about 2 oz.) for the tahini (as that's what was on hand), we served it up on some grilled rosemary bread and scattered the tops with a few snips of chives. We ate it alongside small bowls of chunky tomato soup, curled on the sofa, quite contented. It really was the perfect thing for the cool, early fall evening.
Thanks to both Lucy and Sophie for the inspired suggestion and the perfectly-timed recipe!