You'll never run into an odder-looking vegetable than romanesco (except perhaps kohlrabi). A member of the big and infinitely varied brassica family, it has sort of a mild broccoli flavor, with a texture similar to cauliflower. It is also one of those vegetables that you'll likely only find locally for a short time in late fall or early winter. Romanesco takes almost twice as long to grow to maturity as it's broccoli and cauliflower cousins; it does not like hot weather, it does not care for freezing temperatures; it doesn't really do well in the lengthening days of spring. I've tried growing it at other times of the year with absolutely no luck - this is one finicky plant that definitely has a 'Goldilocks spot!'
To cook, use a small paring knife to break the heads up into florets. Romanesco cooks up beautifully with much the same treatment you'd give cauliflower. Our favorite methods are steamed, raw, and especially roasted. While you're waiting for your romanesco to cook, give yourself an easy little math lesson on fractals to further appreciate the beauty of this weird-looking vegetable!