Heat the oil over high heat in a large skillet. When the oil starts to smoke, add the mustard seeds and immediately cover with a lid or a spatter screen. When the seeds begin to stop popping, carefully add in the garlic and turmeric (you may wish to remove the skillet momentarily from the heat to do this to help the oil cool down a bit faster to keep the seeds from burning) and lower the heat to medium heat. Cook, stirring frequently, for a minute or two to soften the garlic.
Add the cabbage, salt, and cayenne and give the mixture a good stir to coat the cabbage in the oil and spices. Cover and cook until the cabbage is tender-crisp or completely soft, as desired.
Taste and adjust salt if necessary. Serve hot.