Break up the cauliflower into florets that are about 4cm/1½ inches across at the head and 4–5cm/1½–2 inches in length. Let them soak in a bowl of water for 30 minutes. Drain.
Put the onion and ginger into a blender with 4 tablespoons of the measured water. Blend until you have a paste.
Heat the oil in a 23–25cm/9–10 inch wide pot or deep frying pan ov