While the cauliflower’s cooking, prepare the batter. Cut a few 0.5cm rings off one end of the onion and set aside (these will go on top of the cake); coarsely chop the rest. Heat the oil in a pan and on a low heat sauté the chopped onion and rosemary for eight minutes. Remove from the heat, leave to cool down, add the eggs and basil, and whisk.