Choose a sauté pan that can later accommodate all the meat and the cauliflower, put in the olive oil, and turn on the heat to medium high. As soon as the oil is hot enough that it sizzles instantly when you put in a piece of meat, slip in the lamb, a few pieces at a time. Brown the meat on one side, then turn it, and when it has browned well on all sides, transfer it to a platter using a slotted spoon or spatula.
Add the chopped onion to the pan and cook it, stirring frequently, until it has become colored a medium gold. Add the garlic and cook it, stirring, just until its scent begins to rise. Before its color deepens, return all the lamb pieces to the pan.
Add the rosemary, fennel seeds, salt, and liberal grindings of black pepper. Using a wooden spoon, turn over all ingredients two or three times to coat them well.
Add the white wine, let it bubble briskly for a few seconds, then turn the heat down to very low and put a lid on the pan. Cook, turning the contents of the pan over from time to time, for about 1½ hours at a very gentle but steady simmer, until the meat feels very tender when prodded with a fork.
While the lamb is cooking, trim away the tough outer leaves of the cauliflower, wash the head in cold water, then cut it into little clumps about the size of a small potato.
When the meat is tender, add the cut-up cauliflower, sprinkle it with salt, add the tomatoes, turn over all the ingredients two or three times, then cover the pan again and continue cooking for about 8 minutes or so, until the cauliflower feels tender when tested with a fork. If at any time during the entire cooking cycle you find that the pan juices have dried up and the food is likely to stick to the bottom, add 2tablespoons of water. If, on the contrary, when the meat and cauliflower are both done you find that the cooking juices are too runny, uncover the pan, turn up the heat, and boil them down.
When the cauliflower is done, add the orange zest, turn over the contents of the pan once or twice, then transfer to a warm platter and serve at once.