Separate and cut the cauliflower into 1-inch florets. Peel the cauliflower stem and cut into thin slices. Set aside.
Trim the scallions and chop them, including the entire green part. Set aside.
Measure out the spices and place them, as well as the water, right next to the stove.
Heat the oil in a wok or a saute an over high heat. When the oil is hot, add the mustard, cumin, fennel. Keep a pot lid handy since the seeds may splatter and sputter when added. When the seeds stop sputtering, add the turmeric and immediately add the cauliflower.
Stir-fry the cauliflower until it's evenly coated with spice-infused oil. Add the scallions, salt, and water; mix and cover with a lid. Cook over medium heat and toss a couple times until the cauliflower until the cauliflower is soft, about 10 minutes. Uncover, fold in the coriander, and continue stir-frying until excess moisture evaporates and the cauliflower looks glazed, about 5 minutes. Turn off the heat and serve.