In a large pot, cook the onion in the oil over medium heat until softened. Stir in the turmeric and cayenne and cook for a few seconds to bloom the spices in the oil.
Add the cauliflower, potato and apples. Stir together, and cook for a minute or two.
Add 5 cups water (or vegetable stock), coconut milk, 2 teaspoons salt and black pepper to taste. Bring to a simmer, then lower the heat and partially cover the pan. Cook 25 minutes, or until the vegetables are tender when pierced.
Let the mixture cool slightly. Scoop out and reserve 1 cup cauliflower florets, then puree the rest of the soup in a blender in 2 or 3 batches, or directly in the pot with an immersion blender. You can add more liquid if necessary if it seems too thick. Stir in the reserved cauliflower florets.
To serve, ladle into bowls and top with a spoonful each of Simple Green Sauce and yogurt. Sprinkle with sesame seeds.