Add the cauliflower to a food processor, and blitz until it resembles rice.
Heat the oil in a large frying pan, and add the cauliflower 'rice'. Cook over a fairly high heat for several minutes, stirring occasionally, until the cauliflower starts to turn golden brown. Add the garlic and cook for 1 more minute.
Turn the heat down to medium-low, and add the cheddar, mozzarella, milk, and half of the chives. Mix well until the cheeses have melted.
Season generously, and serve topped with the remaining chives.