Add the cauliflower florets, chopped stems, and onion to the Dutch oven. Pour in the wine and chicken broth (or water), the teaspoon salt, several generous twists of black pepper, and rosemary sprig. Stir everything together, then cover the pan and adjust the heat so the mixture simmers nicely. Cook until the cauliflower is tender, about 30-40 minutes, stirring and smashing with a wooden spoon or spatula a few times as you cook.