Heat the oil in a soup pot over medium heat. Add the onion, 2 cups of the cauliflower and the chopped apple and cook 3 to 4 minutes. Mix in the curry powder and flour and cook 1-2 minutes more. Slowly pour in, stirring steadily, the chicken stock. Mix in the coconut milk. Bring to a boil, and then reduce heat to a gentle simmer. Simmer 10 minutes, or until the cauliflower is quite tender. Puree the soup in a food processor and then return to the pot. Bring back to a simmer; thin with a little more stock if too thick. Add the remaining cauliflower florets and simmer until they are tender, about 5 minutes. Season the soup with salt and garnish bowls of it with thin slices of apple, if desired.