Gently fry the onion in the oil until wilted and transparent. Stir in the curry paste and the cauliflower. When this is well coated, sprinkle with flour, stir until the flour is absorbed, and then add the stock. Bring to the boil and simmer until the vegetables are soft.
Remove from the heat, and put in a blender with the coconut milk. Blend until smooth, and return the soup to the pan.