A bollito misto for a winter supper will include more robust vegetables and a robust sauce to match, in this case a mustard butter. Look to spiraled broccoli Romanesco and cauliflower, cut into bite-sized florets; storage turnips, thickly peeled and cut into wedges; thin rounds of peeled Delicata squash; buttery yellow rutabagas, also thinly sliced; and pear onions. Broccoli stems, once peeled, make a delicate and gorgeous addition. Leeks, celery, fennel, and boiling potatoes, which are found in both seasons, can be included as well. Simmer the vegetables in water with the aromatics—they will take a little longer than summer vegetables—then serve them with this butter dabbed over the top.